Origin and history of antonym

"word having the same sense as another," early 15c., synoneme, sinonyme, from Old French synonyme (12c.) and directly from Late Latin synonymum, from Greek synōnymon "word having the same sense as another," noun use of neuter of synōnymos "having the same name as, synonymous," from syn- "together, same" (see syn-) + onyma, Aeolic dialectal form of onoma "name" (from PIE root *no-men- "name").

Opposite of antonym. In Middle English also as the name of a 7c. book by Isidore, a dialogue between Man and Reason who describe differently the same situations. Until 18c. usually in plural form, synonymes, or, if singular, synonyma. The modern form of the word "scarcely makes its appearance, except in dictionaries, till the close of the 18th century" [OED, 2nd edition, print, 1989].

But what we have long and loosely called synonyms are now understood to be, with trifling exceptions, pseudo-synonyms, by which are meant vocables approximating in import, but not equipollent or interchangeable. [Fitzedward Hall, "Modern English," 1873]

Hall's choice as a technically exact word in place of synonyms was homœosemants.

word-forming element of Greek origin meaning "against, opposed to, opposite of, instead," shortened to ant- before vowels and -h-, from Old French anti- and directly from Latin anti-, from Greek anti (prep.) "over, against, opposite; instead, in the place of; as good as; at the price of; for the sake of; compared with; in opposition to; in return; counter-," from PIE *anti "against," also "in front of, before" (from root *ant- "front, forehead," with derivatives meaning "in front of, before"), which became anti- in Italian (hence antipasto) and French.

It is cognate with Sanskrit anti "over, against," and Old English and- (the first element in answer). A common compounding element in Greek, in some combinations it became anth- for euphonic reasons. It appears in some words in Middle English but was not commonly used in English word formations until modern times. In a few English words (anticipate, antique) it represents Latin ante.

In noun compounds where it has the sense of "opposed to, opposite" (Antichrist, anti-communist) the accent remains on the anti-; in adjectives where it retains its old prepositional sense "against, opposed to," the accent remains on the other element (anti-Christian, anti-slavery).

Proto-Indo-European root meaning "name."

It might form all or part of: acronym; allonym; ananym; anonymous; antonomasia; antonym; binomial; caconym; cognomen; denominate; eponym; eponymous; heteronym; homonym; homonymous; hyponymy; ignominious; ignominy; innominable; Jerome; matronymic; metonymy; metronymic; misnomer; moniker; name; nomenclature; nominal; nominate; noun; onomastic; onomatopoeia; paronomasia; paronym; patronym; patronymic; praenomen; pronoun; pseudonym; renown; synonym; synonymy; synonymous; toponym.

It might also be the source of: Sanskrit nama; Avestan nama; Greek onoma, onyma; Latin nomen; Old Church Slavonic ime, genitive imene; Russian imya; Old Irish ainm; Old Welsh anu "name;" Old English nama, noma, Old High German namo, Old Norse nafn, Gothic namo "name."