late 14c., opposicioun, an astrological term for the situation of two heavenly bodies exactly across from one another in the heavens, from Old French oposicion (12c.) and directly from Latin oppositionem (nominative oppositio) "act of opposing, a placing against," noun of action from past-participle stem 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.)).
General sense of "the position of that which faces or confronts something else" is from c. 1400. The meaning "that which is opposite something else" is from 1540s; meaning "act of resisting, antagonism" is attested from 1580s; sense of "
body of opposers," especially "the political party opposed to the one in power" is from 1704.