An Etymological Dictionary of Astronomy and Astrophysics

In opposition to; contrary to; adverse or hostile to.

Etymology (EN): M.E. agens, ageynes, from ageyn, ongean “again,” from on + gegn “against, toward,” → gegenschein.

Etymology (PE): Patâr from Mid.Pers. paitiyârak, “every thing that counteracts and checks the good powers: adversaries of the evil powers;” Av. paiti-ār- “to advance against,” paiti-ārəna- “adversary,” from paiti- “against,” → counter-, + ar- “to move,” → access; cf.
Mod.Pers. patyâré “affliction, misery, misfortune; anything ugly, terrific;” Mid.Pers. padirag, patirak “against, counter-,” Sogd. patrêt “against, opposite, toward.”