Online Etymology Dictionary

undoubted (adj.): mid-15c., from un- (1) "not" + past participle of doubt (v.).
undress (v.): 1590s, "to shed one's clothing," from un- (2) + dress (v.). Transferred sense of "to strip off (someone's) clothing" is recorded from 1610s. The noun meaning "state of partial or incomplete dress" is attested from 1680s. Undressed "naked (or nearly so)" is recorded from 1610s.
undue (adj.): late 14c., "not owing or payable," from un- (1) "not" + past participle of due (adj.). Formed on model of Old French indeu, Latin indebitus. Meaning "not appropriate, unseasonable" is recorded from late 14c. Sense of "unjustifiable" is attested from c.1400 (implied in unduly). Meaning "excessive" is first recorded 1680s.
undulant (adj.): 1830, from Latin undulans, from unda "wave" (see water).
undulate (v.): 1660s, from undulation. Related: undulated, undulating.
undulation (n.): 1640s, from Medieval Latin *undulatio, from Late Latin undulatus "wavy, undulated," from undula "wavelet," diminutive of Latin unda "wave" (see water).
unduly (adj.): late 14c., from un- (1) "not" + duly.
undying (adj.): c.1300, "immortal," from un- (1) "not" + present participle of die (v.). Figurative sense, of feelings, etc., is recorded from c.1765.
une: "to unite," c.1400, from Latin unire, from unus "one" (see one).
unearned (adj.): c.1200, from un- (1) "not" + past participle of earn. Unearned income is recorded from 1889.
unearth (v.): "to dig up," mid-15c., from un- (2) + earth (v.) "to bury in the ground" (see earth). Related: Unearthed; unearthing.
unearthly (adj.): 1610s, "heavenly, sublime," from un- (1) "not" + earthly. Sense of "ghostly, weird" first recorded 1802. Related: Unearthliness.
uneasy: late 13c., "not comforting," from un- (1) "not" + easy. Meaning "disturbed in mind" is attested from 1670s.
uneducated (adj.): 1580s, from un- (1) "not" + past participle of educate.
unemancipated (adj.): 1775, from un- (1) "not" + past participle of emancipate.
unemotional (adj.): 1876, from un- (1) "not" + emotional. Related: Unemotionally.
unemployed (adj.): 1600, "at leisure, not occupied," from un- (1) "not" + past participle of employ. Meaning "temporarily out of work" is from 1660s. The noun meaning "unemployed persons collectively" is from 1782; unemployment first recorded 1888.
[Say the] voices of the unemployed ...
No man has hired us
With pocketed hands
And lowered faces
We stand about in open places
And shiver in unlit rooms ...

[T.S. Eliot, "Choruses from the Rock"]
unemployment (n.): 1888, from un- (1) "not" + employment.
unencumbered (adj.): 1722, from un- (1) "not" + past participle of encumber.
unending (adj.): 1660s, from un- (1) "not" + present participle of end.
unendurable (adj.): 1620s, from un- (1) "not" + endurable. Related: Unendurably.
unenlightened (adj.): 1660s, "not lit up," from un- (1) "not" + past participle of enlighten. Meaning "not mentally illuminated" is attested from 1650s.
unenthusiastic (adj.): 1805, from un- (1) "not" + enthusiastic. Related: Unenthusiastically.
unenviable (adj.): 1640s, from un- (1) "not" + enviable. Related: Unenviably.
unequal (adj.): 1530s, "unjust, unfair," from un- (1) "not" + equal (adj.). Meaning "not the same in amount, size, quality, etc." is recorded from 1560s (inequal in this sense is from late 14c.). Sense of "inadequate, insufficient" (to some task) is attested from 1690s.
unequivocal (adj.): 1784, from un- (1) "not" + equivocal. Related: Unequivocally.
unerring (adj.): 1640s (implied in unerringly), from un- (1) "not" + gerundive of err. Related: Unerringly.
UNESCO: acronym from United Nations Educational, Scientific and Cultural Organization, which was created in 1945.
unethical (adj.): 1871, from un- (1) "not" + ethical. Related: Unethically.
uneven (adj.): Old English unefen "unequal," from un- (1) "not" + even (adj.). Cf. Old Frisian oniovn, Middle Dutch oneven, Old High German uneban, German uneben, Old Norse ujafn. Meaning "irregular, broken, rugged" (in reference to terrain, etc.) is recorded from late 13c.
uneventful (adj.): 1800, from un- (1) "not" + eventful. Related: Uneventfully.
unevitable (adj.): from un- (1) "not" + evitable. The usual word is inevitable.
unexamined (adj.): late 15c., from un- (1) "not" + past participle of examine.
unexceptionable (adj.): 1660s, from un- (1) "not" + exceptionable.
unexcusable (adj.): late 14c., from un- (1) "not" + excusable. The usual word is inexcusable. Related: Unexcusably.
unexpected (adj.): 1580s, from un- (1) "not" + past participle of expect. Related: Unexpectedly.
unexplainable (adj.): 1711, from un- (1) "not" + explain + -able. Related: Unexplainably.
unexplained (adj.): 1721, from un- (1) "not" + past participle of explain.
unexpurgated (adj.): 1882, from un- (1) "not" + past participle of expurgate.
unfailing (adj.): late 14c., "never coming to an end, unceasing," from un- (1) "not" + gerundive of fail (v.). Related: Unfailingly.
unfair (adj.): Old English unfægr "unlovely," from un- (1) "not" + fair. Cf. Old Norse ufagr, Gothic unfagrs. Meaning "wicked, evil, bad" is recorded from c.1300. Sense of "not equitable, unjust" is first recorded 1713. Related: Unfairly; unfairness.
unfaithful (adj.): mid-14c., "acting falsely," from un- (1) "not" + faithful. In Middle English it also had a sense of "infidel, unbelieving, irreligious" (late 14c.). Sense of "not faithful in marriage" is attested from 1828. Related: Unfaithfully; unfaithfulness.
unfaltering (adj.): 1660s, from un- (1) "not" + present participle of falter. Related: Unfalteringly.
unfamiliar (adj.): 1590s, from un- (1) "not" + familiar. Related: Unfamiliarly.
unfashionable (adj.): 1560s, "incapable of being shaped," from un- (1) "not" + fashionable. Meaning "not in accordance with prevailing fashion" is attested from 1640s. Related: Unfashionably.
unfasten (v.): early 13c., from un- (2) + fasten. Related: Unfastened; unfastening.
unfathomable (adj.): 1610s, originally in the figurative sense, of feelings, conditions, etc., from un- (1) "not" + fathomable. Literal sense attested from 1670s. Related: Unfathomably.
unfathomed (adj.): 1620s, from un- (1) "not" + past participle of fathom.
unfavorable (adj.): mid-15c. (implied in unfavorably), from un- (1) "not" + favorable. Related: Unfavorably.
"We must not indulge in unfavorable views of mankind, since by doing it we make bad men believe that they are no worse than others, and we teach the good that they are good in vain." [Walter Savage Landor, "Imaginary Conversations"]
unfazed (adj.): 1933, American English, from un- + past participle of faze.