unchecked (adj.)
late 15c., from un- (1) "not" + past participle of check (v.).
unchivalrous (adj.)
1814, from un- (1) "not" + chivalrous. Related: Unchivalrously; unchivalric.
unchristian (adj.)
1550s, "not professing Christianity" (of persons), from un- (1) "not" + Christian (adj.). Meaning "at variance with Christian principles" (of actions) is recorded from 1580s.
unchurched (adj.)
1680s, from un- (1) "not" + churched "committed or belonging to a church" (see church (v.)). A verb, unchurch "to remove or exclude (someone) from membership in a church" is recorded from 1610s.
uncial (adj.)
1640s, "pertaining to an ounce," from Latin uncialis "of an inch, of an ounce," from uncia "a twelfth part" (see inch (n.1)). In reference to letters, it is attested from 1712, from Late Latin litterae unciales (Jerome), probably meaning "letters an inch high," from Latin uncialis "of an inch, inch-high." As a noun, "an uncial letter," from 1775.
uncirculated (adj.)
1749, from un- (1) "not" + past participle of circulate (v.).
uncircumcised (adj.)
late 14c., from un- (1) "not" + past participle of circumcise (v.).
uncivil (adj.)
1550s, "barbarous," from un- (1) "not" + civil (adj.). Meaning "impolite" is 1590s.
uncivilized (adj.)
c.1600, "barbarous," from un- (1) "not" + past participle of civilize (v.). Uncivil in the same sense is recorded from 1550s.
unclasp (v.)
1520s, from un- (2) "reverse, opposite of" + clasp (v.). Related: Unclasped; unclasping.
unclassified (adj.)
1813, from un- (1) "not" + past participle of classify.
uncle (n.)
late 13c., from Old French oncle, from Latin avunculus "mother's brother" ("father's brother" was patruus), literally "little grandfather," diminutive of avus "grandfather," from PIE root *awo- "grandfather, adult male relative other than one's father" (cognates: Armenian hav "grandfather," Lithuanian avynas "maternal uncle," Old Church Slavonic uji "uncle," Welsh ewythr "uncle").

Replaced Old English eam (usually maternal; paternal uncle was fædera), which represents the Germanic form of the root (cognates: Dutch oom, Old High German oheim "maternal uncle," German Ohm "uncle").

Also from French are German, Danish, Swedish onkel. As a familiar title of address to an old man, attested by 1793; in the U.S. South, especially "a kindly title for a worthy old negro" [Century Dictionary]. First record of Dutch uncle (and his blunt, stern, benevolent advice) is from 1838; Welsh uncle (1747) was the male first cousin of one's parent. To say uncle as a sign of submission in a fight is North American, attested from 1909, of uncertain signification.
Uncle Sam (n.)
symbol of the United States of America, 1813, coined during the war with Britain as a contrast to John Bull, and no doubt suggested by the initials U.S. in abbreviations. "[L]ater statements connecting it with different government officials of the name of Samuel appear to be unfounded" [OED]. The common figure of Uncle Sam began to appear in political cartoons c.1850. Only gradually superseded earlier Brother Jonathan (1776), largely through the popularization of the figure by cartoonist Thomas Nast. British in World War I sometimes called U.S. soldiers Sammies.
Uncle Tom (n.)
"servile black man," 1922, somewhat inaccurately in reference to the humble, pious, but strong-willed main character in Harriet Beecher Stowe's novel, "Uncle Tom's Cabin" (1852). The image implied in the insult perhaps is more traceable to the late 19c. minstel show versions of the story, which reached a far wider audience than the book.
I don't recall anyone in the 1920s using the term 'Uncle Tom' as an epithet. But what's amazing is how fast it caught on (in the 1930s). Black scholars picked up (the term) and just started throwing it at each other. [Ernest Allen, quoted in Hamilton, Kendra, "The Strange Career of Uncle Tom," Black Issues in Higher Education, June 2002]
As a verb, attested from 1937.
unclean (adj.)
Old English unclæne, "morally impure, defiled, unfit for food," from un- (1) "not" + clean (adj.). Literal sense of "dirty" is recorded from mid-13c.
uncleanly (adj.)
Old English unclænlic; see un- (1) "not" + cleanly (adj.). Related: Uncleanliness.
uncleanness (n.)
Old English unclænnes; see unclean + -ness.
unclear (adj.)
c.1300, "not easy to understand," from un- (1) "not" + clear (adj.). Similar formation in Middle Dutch onclaer, Dutch onklaar, German unklar, Old Norse uklarr, Danish uklar, Swedish oklar. Of persons, in sense of "uncertain, doubtful," it is recorded from 1670s.
uncleared (adj.)
1630s in reference to debts, 1772 in reference to land; from un- (1) "not" + past participle of clear (v.).
unclog (v.)
c.1600, from un- (2) "reverse, opposite of" + clog (v.). Related: Unclogged; unclogging.
unclothe (v.)
c.1300, uncloþe (transitive), from un- (2) "opposite of" + clothe (v.). Reflexive sense is attested from late 14c. Related: Unclothed; unclothing.
unclouded (adj.)
1590s, from un- (1) "not" + past participle of cloud (v.).
uncoil (v.)
1713 (transitive), from un- (2) "reverse, opposite of" + coil (v.). Related: Uncoiled; uncoiling.
uncollectable (adj.)
see uncollectible.
uncollected (adj.)
1730, of things, from un- (1) "not" + past participle of collect (v.).
uncollectible (adj.)
1819, from un- (1) "not" + collectible. Form uncollectable is attested by 1796.
uncolored (adj.)
also uncoloured, 1530s, from un- (1) "not" + past participle of color (v.). As a verb, from uncolor is recorded from early 15c.
uncomely (adj.)
c.1200, "improper, unseemly, indecent," from un- (1) "not" + comely. Related: Uncomeliness.
uncomfortable (adj.)
early 15c. "causing bodily or mental discomfort, affording no comfort," from un- (1) "not" + comfortable (adj.). Intransitive meaning "feeling discomfort, ill-at-ease" is attested from 1796. Related: Uncomfortably.
uncommitted (adj.)
late 14c., "not delegated," from un- (1) "not" + committed. Meaning "not pledged to any particular course or party" is attested from 1814.
uncommon (adj.)
1540s, "not possessed in common," from un- (1) "not" + common (adj.). Meaning "not commonly occurring, unusual, rare" is recorded from 1610s. Related: Uncommonly.
uncommunicative (adj.)
1690s, from un- (1) "not" + communicative.
uncomparable (adj.)
late 14c., "incomparable," from un- (1) "not" + comparable. Meaning "unable to be compared (to something else)" is from 1826. Related: Uncomparably.
uncompassionate (adj.)
1590s, from un- (1) "not" + compassionate (adj.).
uncompensated (adj.)
1774, "not compensated by any good," from un- (1) "not" + past participle of compensate (v.). Meaning "not recompensed" is attested from 1830.
uncomplaining (adj.)
1744, from un- (1) "not" + past participle of complain (v.).
uncomplicated (adj.)
1724, from un- (1) "not" + complicated.
uncompounded (adj.)
1580s, from un- (1) "not" + past participle of compound (v.).
uncomprehending (adj.)
1795, from un- (1) "not" + present participle of comprehend (v.). Related: Uncomprehendingly.
uncomprehensible (adj.)
late 14c., from un- (1) "not" + comprehensible. The usual word is incomprehensible.
uncompromised (adj.)
1775, from un- (1) "not" + past participle of compromise (v.).
uncompromising (adj.)
1799, from un- (1) "not" + present participle of compromise (v.). Related: Uncompromisingly.
unconcerned (adj.)
1630s, from un- (1) "not" + past participle of concern (v.). Related: Unconcernedly.
unconditional (adj.)
1660s, from un- (1) "not" + conditional (adj.). Related: Unconditionally. Unconditional surrender in the military sense is attested from 1730; in U.S., often associated with Civil War Gen. Ulysses S. Grant and the taking of Fort Donelson.
The ringing phrase of Grant's latest despatch circulated through the North like some coinage fresh from the mint, and "Unconditional Surrender," which suited the initials of his modest signature, became like a baptismal name. [James Schouler, "History of the United States of America," Dodd, Mead & Co., 1899].
unconditioned (adj.)
1630s, from un- (1) "not" + past participle of condition (v.).
unconfined (adj.)
c.1600, from un- (1) "not" + past participle of confine (v.).
unconfirmed (adj.)
1560s, "not having received the rite of confirmation," from un- (1) "not" + confirmed. Meaning "not supported by further evidence" is attested from 1670s.
unconformable (adj.)
1590s, from un- (1) "not" + past participle of conformable (v.).
unconformity (n.)
c.1600, from un- (1) "not" + conformity. Geological sense is from 1829.
uncongenial (adj.)
1749, from un- (1) "not" + congenial (adj.).