unseat (v.)
1590s, "to throw down from a seat" (especially on horseback), from un- (2) "reverse, opposite of" + seat (v.). Meaning "to deprive of rank or office" is attested from 1610s; especially of elected office in a representative body from 1834. Related: Unseated; unseating.
unsecure (adj.)
1630s, from un- (1) "not" + secure (adj.). A useful differentiation from insecure since the latter word acquired a psychological sense.
unsecured (adj.)
1780, from un- (1) "not" + past participle of secure (v.).
unseeded (adj.)
1775, "not sown," from un- (1) "not" + past participle of seed (v.). From 1932 in the sports sense.
unseemly (adj.)
early 14c., "unfitting, indecent," from un- (1) "not" + seemly (adj.). Similar formation in Old Norse usoemiligr. Related: Unseemliness.
unseen (adj.)
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.)
1690s, from un- (1) "not" + selfish (adj.). Similar formation in Danish uselvisk, Swedish osjälfvisk. Related: Unselfishly; unselfishness.
unsentimental (adj.)
1752, from un- (1) "not" + sentimental (adj.). Related: Unsentimentally.
unseparable (adj.)
late 14c. from un- (1) "not" + separable (adj.). The usual word now is inseparable.
unsettle (v.)
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.)
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.)
"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.)
1610s; see un- (1) "not" + shake (v.) + -able. Of beliefs, etc., from 1670s.
unshaken (adj.)
mid-15c., "not agitated," from un- (1) "not" + shaken. Meaning "not moved from a firm position" is recorded from 1540s.
unsheathe (v.)
late 14c., from un- (2) "reverse, opposite of" + sheathe (v.). Related: Unsheathed; unsheathing.
unshod (adj.)
"without shoes," Old English unscod; see un- (1) "not" + shod (adj.). Old English also had a verb unscogan "to unshoe."
unshorn (adj.)
Old English unscoren; see un- (1) "not" + shorn (adj.).
unsightly (adj.)
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.)
1650s, from un- (1) "not" + sinkable (see sink (v.)).
unskilled (adj.)
1580s, from un- (1) "not" + skill.
unskillful (adj.)
also unskilful, late 14c., "foolish," from un- (1) "not" + skillful (adj.). Meaning "inexpert" is from 1560s. Related: Unskillfully.
unsmiling (adj.)
1730, from un- (1) "not" + present participle of smile (v.).
unsociable (adj.)
c.1600, from un- (1) "not" + sociable (adj.).
unsold (adj.)
mid-14c., from un- (1) "not" + past participle of sell (v.).
unsolicited (adj.)
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.)
1660s, from un- (1) "not" + past participle of solve (v.).
unsophisticated (adj.)
1620s, "unmixed," from un- (1) "not" + sophisticated (adj.). Meaning "ingenuous, natural, inexperienced" is recorded from 1660s.
unsorted (adj.)
1530s, from un- (1) "not" + past participle of sort (v.).
unsought (adj.)
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.)
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.)
"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.)
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.)
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.)
late 14c., from un- (1) "not" + past participle of speak (v.). Similar formation in Middle Dutch ongesproken, Middle Low German ungesproken.
unsporting (adj.)
1822, from un- (1) "not" + sporting (adj.).
unsportsmanlike (adj.)
1754, from un- (1) "not" + sportsmanlike.
unstable (adj.)
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.)
1836, from un- (2) "reverse, opposite of" + stack (v.). Related: Unstacked; unstacking.
unsteady (adj.)
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.)
"make unsteady," 1530s, from un- (2) "reverse, opposite of" + steady (v.). Related: Unsteadied; unsteadying.
unstick (v.)
1706, from un- (2) "reverse, opposite of" + stick (v.). Related: Unsticking.
unstinted (adj.)
late 15c., from un- (1) "not" + past participle of stint (v.).
unstinting (adj.)
late 14c., "unceasing," from un- (1) "not" + present participle of stint (v.). Meaning "lavish" attested by 1834.
unstop (v.)
"remove the stopper from," late 14c., from un- (2) "reverse, opposite of" + stop (v.). Related: Unstopped; unstopping.
unstoppable (adj.)
1827; see un- (1) "not" + stop (v.) + -able.
unstressed (adj.)
1879, from un- (1) "not" + past participle of stress (v.).
unstring (v.)
1610s, from un- (2) "reverse, opposite of" + string (v.). Related: Unstringing.
unstrung (adj.)
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.)
1840, "unpierced;" 1860, "unglued," from un- (1) "not" + past participle of stick (v.).
unstudied (adj.)
late 14c., "not made a subject of study," from un- (1) "not" + past participle of study (v.). From 1650s as "natural, not artificial."