unsecured (adj.) Look up unsecured at Dictionary.com
1780, from un- (1) "not" + past participle of secure (v.).
unseeded (adj.) Look up unseeded at Dictionary.com
1775, "not sown," from un- (1) "not" + past participle of seed (v.). From 1932 in the sports sense.
unseemly (adj.) Look up unseemly at Dictionary.com
early 14c., "unfitting, indecent," from un- (1) "not" + seemly (adj.). Similar formation in Old Norse usoemiligr. Related: Unseemliness.
unseen (adj.) Look up unseen at Dictionary.com
early 13c., from un- (1) "not" + seen. Similar formation in Old English ungesewen, Middle Dutch ongesien, Dutch ongezien, Old High German ungesëhan, German ungesehen, Old Norse usenn.
unselfish (adj.) Look up unselfish at Dictionary.com
1690s, from un- (1) "not" + selfish (adj.). Similar formation in Danish uselvisk, Swedish osjälfvisk. Related: Unselfishly; unselfishness.
unsentimental (adj.) Look up unsentimental at Dictionary.com
1752, from un- (1) "not" + sentimental (adj.). Related: Unsentimentally.
unseparable (adj.) Look up unseparable at Dictionary.com
late 14c. from un- (1) "not" + separable (adj.). The usual word now is inseparable.
unsettle (v.) Look up unsettle at Dictionary.com
1590s, "undo from a fixed position," from un- (2) "reverse, opposite of" + settle (v.). Of the mind, feelings, etc., attested from 1640s. Related: Unsettled; unsettling.
unsettled (adj.) Look up unsettled at Dictionary.com
1590s, "not peaceful, not firmly established," from unsettle (v.) or else from un- (1) "not" + past participle of settle (v.). Meaning "not occupied by settlers" is attested from 1724.
unsex (v.) Look up unsex at Dictionary.com
"deprive of the qualities considered typical of one's gender," c. 1600, from un- (2) "reverse, opposite of" + sex (n.). Related: Unsexed; unsexing.
unshakable (adj.) Look up unshakable at Dictionary.com
1610s; see un- (1) "not" + shake (v.) + -able. Of beliefs, etc., from 1670s.
unshaken (adj.) Look up unshaken at Dictionary.com
mid-15c., "not agitated," from un- (1) "not" + shaken. Meaning "not moved from a firm position" is recorded from 1540s.
unsheathe (v.) Look up unsheathe at Dictionary.com
late 14c., from un- (2) "reverse, opposite of" + sheathe (v.). Related: Unsheathed; unsheathing.
unshod (adj.) Look up unshod at Dictionary.com
"without shoes," Old English unscod; see un- (1) "not" + shod (adj.). Old English also had a verb unscogan "to unshoe."
unshorn (adj.) Look up unshorn at Dictionary.com
Old English unscoren; see un- (1) "not" + shorn (adj.).
unsightly (adj.) Look up unsightly at Dictionary.com
1540s, "displeasing to the eye," from un- (1) "not" + sight (n.) + -ly (1). Similar formation in Middle Dutch onsichtlijc "invisible; ugly," Middle High German unsihtlih "invisible." Related: Unsightliness.

Middle English sightlie is attested from mid-15c. but only in the sense "visible;" unsightly is attested in Middle English only as an adverb meaning "invisibly" (late 15c.). Sightly as "pleasing to the eye" is from 1560s. Middle English also had unsighty "difficult or displeasing to look at" (early 15c., from sighty "attractive," late 14c.), also unsightily in the same sense (c. 1400).
unsinkable (adj.) Look up unsinkable at Dictionary.com
1650s, from un- (1) "not" + sinkable (see sink (v.)).
unskilled (adj.) Look up unskilled at Dictionary.com
1580s, from un- (1) "not" + skill.
unskillful (adj.) Look up unskillful at Dictionary.com
also unskilful, late 14c., "foolish," from un- (1) "not" + skillful (adj.). Meaning "inexpert" is from 1560s. Related: Unskillfully.
unsmiling (adj.) Look up unsmiling at Dictionary.com
1730, from un- (1) "not" + present participle of smile (v.).
unsociable (adj.) Look up unsociable at Dictionary.com
c. 1600, from un- (1) "not" + sociable (adj.). Insociable is older (1580s) but less common.
unsold (adj.) Look up unsold at Dictionary.com
mid-14c., from un- (1) "not" + past participle of sell (v.).
unsolicited (adj.) Look up unsolicited at Dictionary.com
1580s, "unpetitioned, not approached with a request," from un- (1) "not" + past participle of solicit (v.). Meaning "not asked for, unsought" is from 1680s.
unsolved (adj.) Look up unsolved at Dictionary.com
1660s, from un- (1) "not" + past participle of solve (v.).
unsophisticated (adj.) Look up unsophisticated at Dictionary.com
1620s, "unmixed," from un- (1) "not" + sophisticated (adj.). Meaning "ingenuous, natural, inexperienced" is recorded from 1660s.
unsorted (adj.) Look up unsorted at Dictionary.com
1530s, from un- (1) "not" + past participle of sort (v.).
unsought (adj.) Look up unsought at Dictionary.com
early 13c., from un- (1) "not" + past participle of seek (v.). Similar formation in Middle Dutch ongesocht, Dutch ongezocht, Middle High German ungesuochet, German ungesucht.
unsound (adj.) Look up unsound at Dictionary.com
early 14c., of persons, "diseased, wounded," from un- (1) "not" + sound (adj.). Similar formation in Middle Low German unsund, Middle Dutch ongesont, German ungesund. Meaning "morally corrupt" is recorded from c. 1300; that of "not mentally healthy" is from 1540s. Sense of "not based on reasoning or fact" is attested from 1590s. Related: Unsoundly; unsoundness.
unsparing (adj.) Look up unsparing at Dictionary.com
"showing no mercy," 1580s, from un- (1) "not" + sparing, attested from late 14c. as a present participle adjective from spare (v.). Meaning "profuse" is from 1660s. Related: Unsparingly.
unspeakable (adj.) Look up unspeakable at Dictionary.com
c. 1400, "inexpressible," from un- (1) "not" + speakable (see speak (v.)). Meaning "indescribably bad or wicked" is recorded from mid-15c. Related: Unspeakably.
unspoiled (adj.) Look up unspoiled at Dictionary.com
c. 1500, "not plundered," from un- (1) "not" + past participle of spoil (v.). Meaning "not deteriorated" is attested from 1732. A verb unspoil is attested from c. 1400, but it meant "despoil."
unspoken (adj.) Look up unspoken at Dictionary.com
late 14c., from un- (1) "not" + past participle of speak (v.). Similar formation in Middle Dutch ongesproken, Middle Low German ungesproken.
unsporting (adj.) Look up unsporting at Dictionary.com
1822, from un- (1) "not" + sporting (adj.).
unsportsmanlike (adj.) Look up unsportsmanlike at Dictionary.com
1754, from un- (1) "not" + sportsmanlike.
unstable (adj.) Look up unstable at Dictionary.com
early 13c., "apt to move," from un- (1) "not" + stable (adj.). Similar formation in Middle High German unstabel. Meaning "liable to fall" is recorded from c. 1300; sense of "fickle" is attested from late 13c. An Old English word for this was feallendlic, which might have become *fally.
unstack (v.) Look up unstack at Dictionary.com
1836, from un- (2) "reverse, opposite of" + stack (v.). Related: Unstacked; unstacking.
unsteady (adj.) Look up unsteady at Dictionary.com
1590s, "not firm or secure in position," from un- (1) "not" + steady (adj.). Similar formation in Old Frisian unstadich, German unstätig, Middle Dutch onstadich. Meaning "marked by irregularity" is from 1680s. Related: Unsteadily (1550s).
unsteady (v.) Look up unsteady at Dictionary.com
"make unsteady," 1530s, from un- (2) "reverse, opposite of" + steady (v.). Related: Unsteadied; unsteadying.
unstick (v.) Look up unstick at Dictionary.com
1706, from un- (2) "reverse, opposite of" + stick (v.). Related: Unsticking.
unstinted (adj.) Look up unstinted at Dictionary.com
late 15c., from un- (1) "not" + past participle of stint (v.).
unstinting (adj.) Look up unstinting at Dictionary.com
late 14c., "unceasing," from un- (1) "not" + present participle of stint (v.). Meaning "lavish" attested by 1834.
unstop (v.) Look up unstop at Dictionary.com
"remove the stopper from," late 14c., from un- (2) "reverse, opposite of" + stop (v.). Related: Unstopped; unstopping.
unstoppable (adj.) Look up unstoppable at Dictionary.com
1827; see un- (1) "not" + stop (v.) + -able.
unstressed (adj.) Look up unstressed at Dictionary.com
1879, from un- (1) "not" + past participle of stress (v.).
unstring (v.) Look up unstring at Dictionary.com
1610s, from un- (2) "reverse, opposite of" + string (v.). Related: Unstringing.
unstrung (adj.) Look up unstrung at Dictionary.com
1590s, "with strings relaxed" (of a harp, etc.), from un- (1) "not" + past participle of string (v.). Transferred sense of "weakened, unnerved" is recorded from 1690s.
unstuck (adj.) Look up unstuck at Dictionary.com
1840, "unpierced;" 1860, "unglued," from un- (1) "not" + past participle of stick (v.).
unstudied (adj.) Look up unstudied at Dictionary.com
late 14c., "not made a subject of study," from un- (1) "not" + past participle of study (v.). From 1650s as "natural, not artificial."
unsubscribe (v.) Look up unsubscribe at Dictionary.com
1570s (implied in unsubscribed), from un- (2) "reverse of" + subscribe (v.). Related: unsubscribing.
unsubsidized (adj.) Look up unsubsidized at Dictionary.com
1756, from un- (1) "not" + past participle of subsidize (v.).