unionyoudaoicibaDictYouDict[union 词源字典]
union: [15] Union is one of a range of English words that go back to Latin ūnus ‘one’. This in turn was descended from a prehistoric Indo- European *oinos, which also produced English one. Other members of the family include inch, ounce, unique, unite [15], etc. As for union itself, its immediate ancestor was ūniō, a derivative of ūnus which denoted ‘unity’ or ‘the number one’. The application of the English word to an ‘association of workers’ dates from the early 19th century (a somewhat earlier term was combination).
=> inch, one, ounce, unique, unite[union etymology, union origin, 英语词源]
union (n.)youdaoicibaDictYouDict
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.