unbecoming (adj.)
1590s, from un- (1) "not" + becoming "fitting." Related: Unbecomingly.
unbefitting (adj.)
1580s, from un- (1) "not" + befitting.
unbefriended (adj.)
from un- (1) "not" + past participle of befriend (v.).
unbegotten (adj.)
1530s, from un- (1) "not" + begotten.
unbegun (adj.)
Old English unbegunnen; see un- (1) "not" + begun.
unbeknown (adj.)
"unknown," 1630s, from un- (1) "not" + beknown (see beknow).
unbeknownst (adj.)
1833, vulgar formation from unbeknown (1630s). No clear reason for the -st, but since 19c. this has become the dominant form.
unbelief (n.)
mid-12c., "absence or lack of religious belief; disbelief of the truth of the Gospel," from un- (1) "not" or un- (2) "opposite of" + belief. Old English had ungeleafa in this sense.
unbelievable (adj.)
1540s, from un- (1) "not" + believable. Related: Unbelievably.
unbeliever (n.)
"one who does not believe in any given religion," 1520s, from un- (1) "not" believer. Old English had ungelifend in this sense.
unbend (v.)
mid-13c., "relax a bow by unstringing it," from un- (2) "reverse, opposite of" + bend (v.). Intransitive sense from 1746. Figurative meaning "to become genial, relax" (1748) has a sense opposite to that of unbending "inflexible, obstinate" (1680s), which does not derive from the bow-stringing image.
unbeseeming (adj.)
1580s, "not befitting, inappropriate, unsuitable," from un- (1) "not" + beseeming. A common 17c. word.
unbesought (adj.)
1660s, from un- (1) "not" + besought.
unbias (v.)
"to free from bias," 1708, from un- (2) "reverse, opposite of" + bias (v.).
The truest service a private man may hope to do his country is, by unbiassing his mind as much as possible. [Swift, "The Sentiments of a Church of England Man with respect to Religion and Government," 1708]
unbiased (adj.)
c.1600, literal, in reference to throws at bowls, from un- (1) "not" + biased. Figurative sense of "impartial, unprejudiced" is recorded from 1640s.
unbidden (adj.)
Old English unbedene, "not asked or invited," from un- (1) "not" + bidden. Similar formation in Middle Dutch ongebeden, German ungebeten, Old Norse ubeðinn.
unbind (v.)
Old English unbindan, "to free from binding," from un- (2) "opposite of, reverse" + bind (v.). Similar formation in Old Frisian unbinda, German entbinden, Dutch ontbinden. Literal and figurative senses both present in Old English.
Suæ huæt ðu unbindes ofer eorðu bið unbunden in heofnum. [Lindisfarne Gospels, Matt. xvi:19]
Unbound is from Old English unbunden, in literal sense. Figurative sense first attested late 14c.; of books from 1540s.
unbleached (adj.)
1530s, from un- (1) "not" + past participle of bleach (v.).
unblemished (adj.)
c.1300, from un- (1) "not" + past participle of blemish (v.). Originally in moral sense; material sense is attested from mid-15c.
unblown (adj.)
"not yet bloomed," 1580s, from un- (1) "not" + past participle of blow (v.2).
Life is the rose's hope while yet unblown;
The reading of an ever-changing tale;

[Keats, "Sleep and Poetry"]
unborn (adj.)
Old English unboren "not yet born; stillborn," from un- (1) "not" + born. Similar formation in Old Frisian unbern, Dutch ongeboren, Old High German ungiporan, German ungeboren.
unbosom (v.)
"disclose in confidence" (secret opinions or feelings), 1580s, from un- (1) + bosom.
unbounded (adj.)
1590s, "not limited in extent," from un- (1) "not" + past participle of bound (v.1). Sense of "generous, profuse, liberal" is recorded from 1704. Related: Unboundedness.
unbowed (adj.)
late 14c., "not bent," also figuratively "not subdued," from un- (1) "not" + past participle of bow (v.).
unbreakable (adj.)
late 15c.; see un- (1) "not" + breakable.
unbridled (adj.)
late 14c., originally in figurative sense of "unrestrained, ungoverned," from un- (1) "not" + bridled (see bridle (v.)). Similar formation in Middle Dutch ongebreidelt. Literal sense of "not fitted with a bridle" (of horses) is not recorded before 1550s. The verb unbridle (see un- (2)) is attested from c.1400 in the literal sense; mid-15c. in the figurative sense.
unbroken (adj.)
c.1300, in reference to vows or compacts, from un- (1) "not" + broken. Attested from late 15c. in reference to material things; 1510s in reference to courage, spirit, etc.; 1530s in reference to horses; 1560s in reference to the flow of time. Old English had ungebrocen.
unbuckle (v.)
late 14c., from un- (2) "reverse of" + buckle (v.1). Related: Unbuckled; unbuckling.
unburden (v.)
1530s, "to unload" (transitive), from un- (2) "reverse of" + burden (v.). Similar formation in German entbürden. Reflexive sense is recorded from 1580s. Related: Unburdened; unburdening.
unburied (adj.)
Old English unbyrged "unburied," from un- (1) "not" + past participle of bury (v.).
unbutton (v.)
early 14c., from un- (2) "opposite of" + button (v.). Related: Unbuttoned; unbuttoning.
uncalled (adj.)
c.1400, "not summoned," from un- (1) "not" + past participle of call (v.). Similar formation in Swedish okallad, Danish ukaldet. With for and sense of "unnecessary, intrusive" it is first attested 1610.
uncanny (adj.)
1590s, "mischievous;" 1773 in the sense of "associated with the supernatural," originally Scottish and northern English, from un- (1) "not" + canny.
uncap (v.)
1560s, from un- (2) "reverse, opposite of" + cap (v.). Related: Uncapped; uncapping.
uncaring (adj.)
1786, from un- (1) "not" + caring.
unceasing (adj.)
late 14c., from un- (1) "not" + present participle of cease (v.). Related: Unceasingly (mid-14c.).
uncensored (adj.)
1890, from un- (1) "not" + past participle of censor (v.).
unceremonious (adj.)
1580s, from un- (1) "not" + ceremonious. Related: Unceremoniously.
uncertain (adj.)
c.1300, "of indeterminate time or occurrence," from un- (1) "not" + certain (adj.). Meaning "not fully confident" is recorded from late 14c. (implied in uncertainty). Related: Uncertainly.
uncertainty (n.)
late 14c., from uncertain + -ty.
unchain (v.)
1580s, from un- (2) "opposite of" + chain (v.). Figurative sense of "to liberate" is recorded from 1793. Related: Unchained; unchaining.
unchallenged (adj.)
1630s, from un- (1) "not" + past participle of challenge (v.).
unchangeable (adj.)
mid-14c., from un- (1) + changeable.
unchanged (adj.)
late 14c., from un- (1) "not" + past participle of change (v.).
unchanging (adj.)
1590s, from un- (1) "not" + present participle of change (v.).
uncharacteristic (adj.)
1753, from un- (1) "not" + characteristic (adj.). Related: Uncharacteristically.
uncharitable (adj.)
mid-15c., from un- (1) "not" + charitable (v.). Related: Uncharitably (late 14c.).
uncharted (adj.)
1804, from un- (1) "not" + past participle of chart (v.).
unchartered (adj.)
1805, from un- (1) "not" + past participle of charter (v.).
unchaste (adj.)
late 14c., from un- (1) "not" + chaste.