Online Etymology Dictionary

uninvited (adj.): "not having been invited," 1630s, from un- (1) "not" + past participle of invite (v.).
uninviting (adj.): 1680s, from un- (1) + inviting (adj.).
union (n.): early 15c., "action of joining one thing to another," also "agreement, accord," also "state of matrimony," from Anglo-French unioun, Old French union (12c.), from Late Latin unionem (nominative unio) "oneness, unity, a uniting," also in Latin meaning "a single pearl or onion," from unus "one," from PIE *oi-no- "one, unique" (see one).

Sense of "action of uniting into one political body" is attested from 1540s. Meaning "group of people or states" is from 1650s. Short for trade union, it is recorded from 1833. U.S. political sense is attested from 1775; used especially during the Civil War, in reference to the remainder of the United States after the Southern secession.
Union Jack: 1670s, from union + jack (n.); properly a small British union flag flown as the jack of a ship, but it has long been in use as a general name for the union flag. The Union flag (1630s) was introduced to symbolize the union of the crowns of England and Scotland (in 1603) and was formed of a combination of the cross saltire of St. Andrew and the cross of St. George. The cross saltire of St. Patrick was added 1801 upon the union of parliaments of Great Britain and Ireland.
unionize (v.): 1841, "make into a union" (transitive), from union + -ize. Sense "form into a trade union" is from 1887. Related: Unionized; unionizing.
unipersonal (adj.): 1810, from uni- + personal (adj.).
unipolar (adj.): 1812, originally in electricity, from uni- + polar. Related: Unipolarity.
unique (adj.): c. 1600, "single, solitary," from Middle French unique (16c.), from Latin unicus "only, single, sole, alone of its kind," from unus "one" (see one). Meaning "forming the only one of its kind" is attested from 1610s; erroneous sense of "remarkable, uncommon" is attested from mid-19c. Related: Uniquely; uniqueness.
unisex (adj.): "sexually indistinguishable or neutral," 1968, from uni- + sex (n.).
unisexual (adj.): 1802, "of one sex, having only one sex," from uni- + sexual. Meaning "of or for a single sex" (of schools, etc.) is from 1885. Meaning "unisex" is from 1970. Related: Unisexual.
unison (n.): 1570s, "note having the same pitch as another; identity in pitch of two or more sounds; interval between tones of the same pitch," especially the interval of an octave, from Middle French unisson "unison, accord of sound" (16c.) or directly from Medieval Latin unisonus "having one sound, sounding the same," from Late Latin unisonius "in immediate sequence in the scale, monotonous," from Latin uni- "one" (see one) + sonus "sound" (see sound (n.1)). Figurative sense of "harmonious agreement" is first attested 1640s.
unit (n.): 1560s, "single number regarded as an undivided whole," alteration of unity on the basis of digit. Popularized in John Dee's English translation of Euclid, to express Greek monas (Dee says unity formerly was used in this sense). Meaning "single thing regarded as a member of a group" is attested from 1640s. Extended sense of "a quantity adopted as a standard of measure" is from 1738. Sense of "group of wards in a hospital" is attested from 1893.
unitard (n.): 1961, from uni- "one" + ending from leotard.
unitarian (n.): 1680s, "one who rejects the doctrine of the Trinity," from Modern Latin unitarius (1650s), from Latin unitas (see unity) + -ian. Applied to Muslims and other non-Christian monotheists, but especially (and with a capital -u-) of a Christian body originally founded upon the doctrine of unipersonality. The American Unitarian Association formed in 1825. As an adjective from 1680s.
unitarianism (n.): 1690s, from unitarian + -ism.
unitary (adj.): 1847, "characterized by unity or uniformity;" 1865, "of or relating to a unit;" see unit + -ary.
unite (v.): early 15c. (transitive), from Late Latin unitus, past participle of unire "to unite, make into one" (transitive), from Latin unus "one" (see one). Intransitive sense from 1610s. Related: United; uniting.
united (adj.): "made into one," 1550s, past participle adjective from unite (v.).
United Kingdom: attested from 1706.
United Nations: 1942, "the Allied nations at war with the Axis powers;" the international body (officially the United Nations Organization) was chartered in 1945.
Such negotiation as may occur in New York is not conducted within the walls of the tall building by the East River: it is carried out elsewhere, in accordance with those principles of courtesy, confidence and discretion which must for ever remain the only principles conducive to the peaceful settlement of disputes. [Harold Nicholson, "The Evolution of Diplomatic Method," 1954]
united states (n.): attested from 1617, originally with reference to Holland; the North American confederation first so called in 1776. United Provinces were the seven northern provinces of the Netherlands, allied from 1579, later developing into the kingdom of Holland.
uniter (n.): 1580s, agent noun from unite (v.).
unity (n.): c. 1300, "state or property of being one," from Anglo-French unite, Old French unite "uniqueness, oneness" (c. 1200), from Latin unitatem (nominative unitas) "oneness, sameness, agreement," from unus "one" (see one).
univalve: of mollusks and shells, 1660s (noun and adjective), from uni- + valve.
universal (adj.): late 14c., "pertaining to the whole of something specified; occurring everywhere," from Old French universel "general, universal" (12c.), from Latin universalis "of or belonging to all," from universus "all together, whole, entire" (see universe). In mechanics, a universal joint (1670s) is one which allows free movement in any direction. Universal product code is recorded from 1974.
universalism (n.): 1805 in theology, "the doctrine of universal salvation," from universal (adj.) + -ism. Universalist "one who, professing the Christian faith, believes in the eventual redemption of all humanity" is attested from 1620s.
universality (n.): late 14c., from Old French universelite (14c.) or directly from Late Latin universalitas, from Latin universalis "universal" (see universal).
universally (adv.): late 14c., from universal + -ly (2).
universe (n.): 1580s, "the whole world, cosmos, the totality of existing things," from Old French univers (12c.), from Latin universum "all things, everybody, all people, the whole world," noun use of neuter of adjective universus "all together, all in one, whole, entire, relating to all," literally "turned into one," from unus "one" (see one) + versus, past participle of vertere "to turn" (see versus).
university (n.): c. 1300, "institution of higher learning," also "body of persons constituting a university," from Anglo-French université, Old French universite "universality; academic community" (13c.), from Medieval Latin universitatem (nominative universitas), "the whole, aggregate," in Late Latin "corporation, society," from universus "whole, entire" (see universe). In the academic sense, a shortening of universitas magistrorum et scholarium "community of masters and scholars;" superseded studium as the word for this. The Latin word also is the source of Spanish universidad, German universität, Russian universitetŭ, etc.
univocal (adj.): 1540s, "having one meaning only," from Latin univocus, from uni- (see uni-) + vox (see voice (n.)). Related: Univocally.
unjoin (v.): mid-14c., from un- (2) "opposite of" + join (v.). Related: Unjoined; unjoining.
unjust (adj.): late 14c., of persons, "sinful; perpetrating injustice," from un- (1) "not" + just (adj.). Of actions, from c. 1400. Related: Unjustly.
unjustifiable (adj.): 1640s, from un- (1) "not" + justifiable. Related: Unjustifiably.
unjustified (adj.): mid-14c., "not punished or executed," from un- (1) "not" + past participle of justify (v.). Meaning "not proven to be right or proper" is attested from 1680s.
unkempt (adj.): 1570s, from un- (1) "not" + kempt "well-combed, neat," from variant past participle of Middle English kemben "to comb," from Old English cemban "to comb," from Proto-Germanic *kambijan, from *kamb- "comb" (see comb (n.)). Form unkembed is recorded from late 14c. The verb kemb is rare after 1400s, but its negative past participle form endures.
unkept (adj.): mid-14c., "neglected," from un- (1) "not" + past participle of keep (v.). From late 14c. as "disregarded;" 1842 as "not stored or retained."
unkillable (adj.): 1841, from un- (1) "not" + killable.
unkind (adj.): Old English uncynde "unnatural, not natural;" see un- (1) "not" + kind (adj.). Meaning "lacking in kindness" is recorded from mid-14c.
unkindly (adv.): c. 1200, "unsuitably, improperly," from un- (1) "not" + kindly (adv.). From mid-15c. as "discourteously, maliciously."
unkindly (adj.): early 13c., "not natural, unnatural," from un- (1) "not" + kindly (adj.). From c. 1300 as "without natural affection, unfraternally." Old English had ungecyndelic.
unkindness (n.): c. 1300, "state or character of being unkind, lack of natural regard or proper consideration," from un- (1) "not" + kindness (n.), or from unkind (adj.) + -ness. Meaning "an unkind act" is from c. 1500.
unknit (v.): Old English uncnyttan; see un- (2) "reverse" + knit (v.). Related: Unknitted; unknitting.
unknowable (adj.): late 14c., from un- (1) "not" + knowable (adj.).
unknowing (adj.): c. 1300, "without knowledge, ignorant," from un- (1) "not" + present participle of know (v.). Noun meaning "ignorance" is mid-14c., especially in phrase cloud of unknowing, title of a medieval book of Christian mysticism. Related: Unknowingly. A verb unknow "fail to recognize" is attested from late 14c.
unknown (adj.): c. 1300, "strange, unfamiliar" (of persons, places), from un- (1) "not" + past participle of know (v.). Compare Old English ungecnawen. In reference to facts, "not discovered or found out," it attested from early 14c. The noun meaning "unknown person" is recorded from 1590s; the unknown "that which is unknown" is from 1650s.
unlace (v.): c. 1300, from un- (2) "opposite of" + lace (v.). Related: Unlaced; unlacing.
unlade (v.): "remove the cargo from," Old English onhladen; see un- (2) "reverse, opposite of" + lade (v.). Related: Unladen; unlading.
unladen (adj.): 1802, past participle adjective from unlade (v.).
unladylike (adj.): 1824, from un- (1) "not" + ladylike.