late 14c., "placed or situated on the other side of (something)," from Old French opposite, oposite "opposite, contrary" (13c.), from Latin oppositus "standing against, opposed, opposite," past participle of opponere "set against," from assimilated form of ob "in front of, in the way of" (see ob-) + ponere "to put, set, place" (see position (n.)).
The meaning "contrary in character, of a totally different nature" is from 1570s. As a noun from late 14c., "the opposite side of" (a place, the body, etc.), "an opposite position or condition." From early 15c. as "that which is opposite in character or quality;" also "an opponent." As a preposition from 1758. As an adverb from 1817. Related: Oppositely.