Online Etymology Dictionary

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.
unbuckle (v.): late 14c., from un- (2) "reverse of" + buckle (v.). Related: Unbuckled; unbuckling.
unburden (v.): 1530s, "to unload" (transitive), from un- (2) "reverse of" + burden (v.). Cf. German entbürden. Reflective sense is recorded from 1580s. Related: Unburdened; unburdening.
unburied (adj.): Old English unbyrged, 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.). Cf. 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.
uncaring (adj.): 1786, from un- (1) "not" + caring.
unceasing (adj.): late 14c., from un- (1) "not" + present participle of cease. 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.
unchangeable (adj.): mid-14c., from un- (1) + changeable.
unchanged: 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.
uncharted (adj.): 1847, from un- (1) "not" + past participle of chart.
unchartered (adj.): 1805, from un- (1) "not" + past participle of charter.
unchaste (adj.): late 14c., from un- (1) "not" + chaste.
unchecked (adj.): late 15c., from un- (1) "not" + past participle of check (v.).
unchivalrous (adj.): 1821, from un- (1) "not" + chivalrous. Related: Unchivalrously. Alternative unchivalric is recorded from 1851.
unchristian (adj.): 1550s, "not professing Christianity" (of persons), from un- (1) "not" + Christian. 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: 1640s, "pertaining to an ounce," from Latin uncialis "of an inch, of an ounce," from uncia "a twelfth part" (see inch). 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."
uncirculated (adj.): 1749, from un- (1) "not" + past participle of circulate (see circulation).
uncivil (adj.): 1550s, "barbarous," from un- (1) "not" + civil. Meaning "impolite" is 1590s.
uncivilized (adj.): c.1600, "barbarous," from un- (1) "not" + past participle of civilize. Uncivil in the same sense is recorded from 1550s.
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," literally "little grandfather," diminutive of avus "grandfather," from PIE root *awo- "grandfather, adult male relative other than one's father" (cf. 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 (cf. Dutch oom, Old High German oheim "maternal uncle," German Ohm "uncle").

Also from French are German, Danish, Swedish onkel. First record of Dutch uncle (and his blunt, stern, benevolent advice) is from 1838; Welsh uncle (1747) was the 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. "[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.
unclear (adj.): c.1300, "not easy to understand," from un- (1) "not" + clear (adj.). Cf. 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 is recorded from 1630s in reference to debts, 1772 in reference to land.
unclothe (v.): c.1300, uncloþe (transitive), from un- (2) + clothe (v.). Refl. sense is attested from late 14c. Related: Unclothed; unclothing.
uncollectable (adj.): 1927, from un- + collectable (see collectible).
uncollectible (adj.): by 1833, from un- + collectible.
uncomfortable (adj.): early 15c. "causing discomfort," from un- (1) "not" + comfortable. 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.
uncompensated (adj.): 1774, "not compensated by any good," from un- (1) "not" + past participle of compensate. Meaning "not recompensed" is attested from 1830.
uncomplicated (adj.): 1724, from un- (1) "not" + complicated.
uncomprehending (adj.): 1795, from un- (1) "not" + present participle of comprehend. 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.
uncompromising (adj.): 1799, from un- (1) "not" + gerundive of compromise (v.). Related: Uncompromisingly.