An Etymological Dictionary of Astronomy and Astrophysics

A person who is on an opposing side in a game, contest, controversy, or the like; adversary (Dictionary.com).

Etymology (EN): L. opponent-, p.p. of opponere “to oppose, to object to,” literally “set against, set opposite,” from op- variant of ob- before p
“against” + ponere “to put, set, place,” → position.

Etymology (PE): Pâdistgar, from pâdist, → opposition, + -gar, → -or.