An Etymological Dictionary of Astronomy and Astrophysics

A combining form meaning “first, foremost, earliest form of,” used in the formation of compound words such as → protogalaxy, → protoplanet, → protostar, etc.

Etymology (EN): From Gk. proto-, combining form of protos “first,” superlative of pro “before,” cognate with O.S. pruvu “first;” Rus. pervyy “first;”
Av. pauruua-, as below.

Etymology (PE): Purvâ-, from O.Pers. paruviya- “former, initial,” Av. pourva-, pouruuiia-, pauruua-, paoiriia- “first, initial, former;” cf. Skt. purva- “first, former, being before;” Tokharian B parwe “first;” PIE base *prwos “first.”