An Etymological Dictionary of Astronomy and Astrophysics

A combining form (hydr- before a vowel) originally meaning “water,” but also “liquid, gas.” In chemical nomenclature, often denotes a compound of hydrogen.

Etymology (EN): Gk. hydro-, combining form of hydor “water,” cognate with Skt. udá- “water;” Khotanese ūtcā “water;” Hittite uātar; L. unda “wave;”
O.C.S., Rus. voda; Lith. vanduo; P.Gmc. *watar (cf. Du. water; O.H.G. wazzar; Ger. Wasser; Goth. wato; O.E. wæter; E. water); from PIE base *wed- “water; wet.”

Etymology (PE): Hidro-, loanword from Gk., as above. Âb- “water,” from
Mid.Pers. âb “water;” O. Pers. ap- “water;” Av. ap- “water;” cf. Skt. áp- “water;”
Hitt. happa- “water;” PIE āp-, ab- “water, river;”
cf. Gk. Apidanos, proper noun, a river in Thessalia; L. amnis “stream, river” (from *abnis); O.Ir. ab “river,” O.Prus. ape “stream,” Lith. upé “stream;” Latv. upe “brook.”