unadulterated (adj.)
1719, from un- (1) "not" + past participle of adulterate (v.).
unadvertised (adj.)
mid-15c., "uninformed, unadvised," from un- (1) + advertised. Sense of "not announced or made known" is from 1864.
unaffected (adj.)
1580s, "not influenced, untouched in mind or feeling," from un- (1) "not" + past participle of affect (v.). Meaning "not adopted or assumed, genuine" is recorded from 1590s; that of "not acted upon or altered (by something)" is first attested 1830. Related: Unaffectedly; unaffectedness.
unafraid (adj.)
early 15c., from un- (1) "not" + afraid.
unaided (adj.)
1660s, from un- (1) "not" + past participle of aid (v.).
unalienable (adj.)
1610s, from un- (1) "not" + alienable. Related: Unalienably.
unalloyed (adj.)
1670s (figurative); 1760s (literal), from un- (1) "not" + past participle of alloy (v.).
unalterable (adj.)
1610s, from un- (1) "not" + alterable. Related: Unalterably.
He reach'd a middle height, and at the stars,
Which are the brain of heaven, he look'd, and sank.
Around the ancient track march'd, rank on rank,
The army of unalterable law.

[George Meredith, "Lucifer in Starlight"]
unaltered (adj.)
1550s, from un- (1) "not" + past participle of alter (v.).
unambiguous (adj.)
1630s, from un- (1) "not" + ambiguous. Related: Unambiguously; unambiguousness.
unanimity (n.)
mid-15c., from Old French unanimite (14c.), from Late Latin unanimitatem (nominative unanimitas) "unanimity, concord," from unanimus (see unanimous).
unanimous (adj.)
1610s, from Latin unanimus "of one mind, in union," from unus "one" (see one) + animus "mind" (see animus). Related: Unanimously.
unannounced (adj.)
1775, from un- (1) "not" + past participle of announce (v.).
unanswerable (adj.)
1610s, "admitting of no answer," from un- (1) "not" + answerable.
unanswered (adj.)
late 14c., from un- (1) "not" + past participle of answer (v.).
unanticipated (adj.)
1741, from un- (1) "not" + past participle of anticipate (v.).
unapologetic (adj.)
1834, from un- (1) "not" + apologetic. Related: Unapologetically.
unappeasable (adj.)
1560s, from un- (1) "not" + appeasable. Related: Unappeasably.
Desolate winds that cry over the wandering sea;
Desolate winds that hover in the flaming West;
Desolate winds that beat the doors of Heaven, and beat
The doors of Hell and blow there many a whimpering ghost;
O heart the winds have shaken, the unappeasable host
Is comelier than candles at Mother Mary's feet.

[W.B. Yeats, "The Unappeasable Host," 1899]
unappreciated (adj.)
1809, from un- (1) "not" + past participle of appreciate (v.).
unappreciative (adj.)
1834, from un- (1) "not" + appreciative. Related: Unappreciatively; unappreciativeness.
unapprehended (adj.)
1590s, "not understood;" 1610s, "not taken or arrested," from un- (1) "not" + past participle of apprehend (v.).
unapproachable (adj.)
1580s, of places, from un- (1) "not" + approachable. Of persons, "distant, aloof," attested from 1848. Related: Unapproachably.
unapproved (adj.)
early 15c., from un- (1) "not" + past participle of approve (v.).
unarmed (adj.)
c.1300, "with armor removed," from un- (1) "not" + armed, or else past participle adjective from unarm "strip of armor" (c.1300), from un- (2) "opposite of" + arm (v.). Meaning "not fitted to attack, weaponless" is from late 14c.
unary (adj.)
1923, from Latin unus "one" (see one) on model of binary, etc.
unashamed (adj.)
c.1500, implied in unashamedness, from un- (1) "not" + ashamed. Related: Unashamedly.
unasked (adj.)
mid-13c., "uninvited," from un- (1) "not" + past participle of ask (v.). Old English had ungeaxod.
unaspiring (adj.)
1680s, from un- (1) "not" + present participle of aspire (v.).
unassailable (adj.)
1590s, from un- (1) "not" + assailable (see assail (v.)). Related: Unassailably.
unassimilated (adj.)
1748, from un- (1) "not" + past participle of assimilate (v.).
unassisted (adj.)
1610s, from un- (1) "not" + past participle of assist (v.). Related: Unassistedly.
unassuming (adj.)
"unpretentious, modest," 1726, from un- (1) "not" + present participle of assume (v.). Related: Unassumingly.
unattached (adj.)
late 15c., "not arrested or seized," from un- (1) "not" + past participle of attach (v.). Meaning "not associated with any body or institution" is recorded from 1796; sense of "single, not engaged or married" is first attested 1874.
unattainable (adj.)
1660s, from un- (1) "not" + attainable.
unattended (adj.)
c.1600, "alone, unaccompanied," from un- (1) "not" + past participle of attend (v.). Meaning "with no one in attendance" is from 1796.
unattested (adj.)
1660s, from un- (1) "not" + past participle of attest (v.).
unattractive (adj.)
1729, from un- (1) "not" + attractive. Related: Unattractively; unattractiveness.
unauthorized (adj.)
1590s, from un- (1) "not" + past participle of authorize (v.).
unavailability (n.)
1855, from un- (2) "opposite of" + availability, or else from unavailable + -ity.
unavailable (adj.)
1540s, "ineffectual," from un- (1) "not" + available. Meaning "incapable of being used" is recorded from 1855. Unavailing has taken up the older sense of the word.
unavailing (adj.)
"ineffectual, useless," 1660s, from un- (1) "not" + availing. Also see unavailable.
unavoidable (adj.)
mid-15c., from un- (1) "not" + avoidable. Related: Unavoidably.
unaware (adj.)
1590a; see unawares.
unawares (adv.)
1530s, "without being aware," from un- (1) "not" + aware + adverbial genitive -s. Meaning "without being noticed" is recorded from 1660s. Form unaware is recorded from 1590s.
unbalance (v.)
1856, from un- (2) "reverse, opposite of" + balance (v.).
unbalanced (adj.)
1640s, from un- (1) "not" + past participle of balance (v.). Earliest use is in reference to the mind, judgment, etc. Of material things, it is recorded from 1732.
unbar (v.)
late 14c., from un- (2) "reverse, opposite of" + bar (v.). Related: Unbarred; unbarring.
unbearable (adj.)
mid-15c., from un- (1) "not" + bearable. Related: Unbearably. Old English had unberendlic.
unbeatable (adj.)
1897, from un- (1) "not" + beatable.
unbeaten (adj.)
late 13c., "not beaten or struck," from un- (1) + beaten. In the sense of "undefeated" it is first recorded 1757. Old English had ungebeaten "unwrought, unstruck."