An Etymological Dictionary of Astronomy and Astrophysics

A prefix meaning “below,” used with second elements of any origin.
→ infrared; → infrasound.

Etymology (EN): From L. infra “below, underneath, beneath,” from PIE *ndher; cf. Mod.Pers. zir “below, down,” akin to → under-.

Etymology (PE): Foru-, from foru “down, downward; below; under, beneath; low,” from Mid.Pers. frôt “down, downward;” O.Pers. fravata “forward, downward;” cf. Skt. pravát- “a sloping path, the slope of a mountain.”