unpleasant (adj.) Look up unpleasant at Dictionary.com
1530s, from un- (1) "not" + pleasant (adj.). Related: Unpleasantly.
unpleasantness (n.) Look up unpleasantness at Dictionary.com
1540s, "state or quality of being unpleasant," from unpleasant + -ness. By 1835 as "a slight quarrel, a minor misunderstanding." The late unpleasantness as a humorously polite Southern description of the American Civil War is attested from 1868.
unplug (v.) Look up unplug at Dictionary.com
1775, from un- (2) "reverse, opposite of" + plug (v.). Related: Unplugged; unplugging.
unpolished (adj.) Look up unpolished at Dictionary.com
late 14c., from un- (1) "not" + past participle of polish (v.). In reference to style, language, etc., attested from late 15c.
unpolluted (adj.) Look up unpolluted at Dictionary.com
c.1600, from un- (1) "not" + past participle of pollute (v.).
unpopular (adj.) Look up unpopular at Dictionary.com
1640s, from un- (1) "not" + popular (adj.). Related: Unpopularly.
unpopularity (n.) Look up unpopularity at Dictionary.com
1735, from un- (1) "not" + popularity (n.).
unpracticed (adj.) Look up unpracticed at Dictionary.com
also unpractised, 1550s, "unexpert," of persons, from un- (1) "not" + past participle of practice (v.). From 1530s as "not done, untried."
unprecedented (adj.) Look up unprecedented at Dictionary.com
1620s, from un- (1) "not" + precedented. In common use from c.1760.
unpredictable (adj.) Look up unpredictable at Dictionary.com
1840, from un- (1) "not" + predictable (adj.). Related: Unpredictably; unpredictability.
unprejudiced (adj.) Look up unprejudiced at Dictionary.com
1610s, from un- (1) "not" + past participle of prejudice (v.).
unpremeditated (adj.) Look up unpremeditated at Dictionary.com
1590s, from un- (1) "not" + past participle of premeditate (v.).
unprepared (adj.) Look up unprepared at Dictionary.com
1540s, from un- (1) "not" + past participle of prepare (v.).
unprepossessing (adj.) Look up unprepossessing at Dictionary.com
1816, from un- (1) "not" + prepossessing (adj.).
unpresentable (adj.) Look up unpresentable at Dictionary.com
1828, from un- (1) "not" + presentable (adj.).
unpretentious (adj.) Look up unpretentious at Dictionary.com
1859, from un- (1) "not" + pretentious (adj.). Related: Unpretentiously; unpretentiousness.
unpreventable (adj.) Look up unpreventable at Dictionary.com
1610s, from un- (1) "not" + preventable (adj.).
unprincipled (adj.) Look up unprincipled at Dictionary.com
1630s, "not instructed" (in something), from un- (1) "not" + principled (adj.). Meaning "not honorable" is recorded from 1640s.
unproductive (adj.) Look up unproductive at Dictionary.com
1756, from un- (1) "not" + productive (adj.). Related: Unproductively; unproductiveness.
unprofessional (adj.) Look up unprofessional at Dictionary.com
1806, from un- (1) "not" + professional (adj.).
unprofitable (adj.) Look up unprofitable at Dictionary.com
early 14c., from un- (1) "not" + profitable (adj.). Related: Unprofitably.
unpromising (adj.) Look up unpromising at Dictionary.com
1660s, from un- (1) "not" + promising (adj.).
unprompted (adj.) Look up unprompted at Dictionary.com
from un- (1) "not" + past participle of prompt (v.).
unpronounceable (adj.) Look up unpronounceable at Dictionary.com
1831, from un- (1) "not" + pronounceable (adj.).
unpropitious (adj.) Look up unpropitious at Dictionary.com
c.1600 (implied in unpropitiously), from un- (1) "not" + propitious (adj.).
unprotected (adj.) Look up unprotected at Dictionary.com
1590s, from un- (1) "not" + past participle of protect (v.).
unprovable (adj.) Look up unprovable at Dictionary.com
early 15c., from un- (1) "not" + provable (adj.).
unproved (adj.) Look up unproved at Dictionary.com
"not demonstrated to be true," 1530s, from un- (1) "not" + past participle of prove (v.).
unprovoked (adj.) Look up unprovoked at Dictionary.com
1580s, from un- (1) "not" + past participle of provoke (v.).
unpublished (adj.) Look up unpublished at Dictionary.com
c.1600, from un- (1) "not" + past participle of publish. In reference to an author, attested from 1934.
unpunctual (adj.) Look up unpunctual at Dictionary.com
1740, from un- (1) "not" + punctual (adj.).
unpunished (adj.) Look up unpunished at Dictionary.com
mid-14c., from un- (1) "not" + past participle of punish (v.).
unqualified (adj.) Look up unqualified at Dictionary.com
1550s, "not having necessary qualifications, from un- (1) "not" + past participle of qualify (v.). Meaning "not modified or limited" is recorded from 1796. As a verb, unqualify "disqualify" is from 1650s.
unquantifiable (adj.) Look up unquantifiable at Dictionary.com
1888, from un- + quantifiable (adj.). Related: Unquantifiably.
unquenchable (adj.) Look up unquenchable at Dictionary.com
late 14c., of fire; 1560s, of thirst, from un- (1) "not" + quench (v.) + -able. Related: Unquenchably.
unquestionable (adj.) Look up unquestionable at Dictionary.com
c.1600, from un- (1) "not" + questionable (adj.). Related: Unquestionably.
unquestioned (adj.) Look up unquestioned at Dictionary.com
c.1600, "not interrogated;" 1620s, "not to be opposed or disputed," from un- (1) "not" + past participle of question (v.).
unquiet (adj.) Look up unquiet at Dictionary.com
1520s, from un- (1) "not" + quiet (adj.).
unquote (v.) Look up unquote at Dictionary.com
1915, in telegraphs, where punctuation had to be spelled out and quote and unquote were used in place of the quotation marks; from un- (2) "reverse, opposite of" + quote (v.). Quote unquote together to indicate quotation of the word or phrase to follow (often with ironic intent) is from 1942.
unravel (v.) Look up unravel at Dictionary.com
c.1600 (transitive), from un- (2) + ravel (v.). Intransitive from 1640s. "The prefix is either reversive or intensive, according as ravel is taken to mean 'tangle' or 'untangle'" [Century Dictionary]. Related: Unravelled; unravelling; unravellment.
unreachable (adj.) Look up unreachable at Dictionary.com
1590s, from un- (1) "not" + reachable (adj.).
unread (adj.) Look up unread at Dictionary.com
mid-15c., from un- (1) "not" + past participle of read (v.).
unreadable (adj.) Look up unreadable at Dictionary.com
1787, of written material, "dull, distasteful," from un- (1) "not" + readable (adj.). Meaning "illegible" is from 1830.
unready (adj.) Look up unready at Dictionary.com
mid-14c., "not prepared," from un- (1) "not" + ready (adj.).

In English history, applied from c.1200 (Etheldredus Unrad) to Anglo-Saxon King Æðelræd II (968-1016), where it preserves Middle English unredi, a different adjective, from Old English ungeræd "ill-advised, rede-less, no-counsel" and plays on the king's name (which means "good-counsel"). Old English ræd "advice, counsel" is related to read (v.). Rede "counsel" survived in poetic usage to 17c. An attempted revival by Scott (19c.) failed, though it is used in Tolkien's "Lord of the Rings."
unreal (adj.) Look up unreal at Dictionary.com
c.1600, "not real," from un- (1) "not" + real (adj.). Meaning "impractical, visionary" is by 1660s. Slang sense of "wonderful, great" is first recorded 1965.
Unreal City,
Under the brown fog of a winter dawn,
A crowd flowed over London Bridge, so many,
I had not thought death had undone so many.

[Eliot, "The Waste Land," 1922]
unrealistic (adj.) Look up unrealistic at Dictionary.com
1865, from un- (1) "not" + realistic. Related: Unrealistically.
unreality (adj.) Look up unreality at Dictionary.com
1751, from un- (1) "not" + reality (n.).
unreason (n.) Look up unreason at Dictionary.com
c.1300, "injustice;" 1827, "absense of reason," from un- (1) "not" + reason (n.).
unreasonable (adj.) Look up unreasonable at Dictionary.com
mid-14c., "irrational, illogical," from un- (1) "not" + reasonable. From late 14c. as "excessive, going beyond what is sensible or realistic." Related: Unreasonably; unreasonableness.
unrecognizable (adj.) Look up unrecognizable at Dictionary.com
1817, from un- (1) "not" + recognizable (see recognize (v.)). Related: Unrecognizably.