An Etymological Dictionary of Astronomy and Astrophysics

1. To act against or provide resistance to.
   

2. To stand in the way of; hinder; obstruct (Dictionary.com).

Etymology (EN): M.E., from O.Fr. oposer “to oppose, resist; contradict,” from poser “to place, lay down,” blended with L. opponere “to oppose, to object,” → position.

Etymology (PE): Pâdistidan, infinitive from pâdist, → opposition.