Old English grene "green, young, immature, raw," earlier groeni, from West Germanic *gronja- (cf. Old Saxon grani, Old Frisian grene, Old Norse grænn, Danish grøn, Dutch groen, Old High German gruoni, German grün), from PIE root *ghre- "grow" (see grass), through sense of "color of living plants."
Meaning "a field, grassy place" was in Old English. Sense of "of tender age, youthful" is from early 15c.; hence "gullible" (c.1600). The color of jealousy at least since Shakespeare (1596); "Greensleeves," ballad of an inconstant lady-love, is from 1570s. Green light in figurative sense of "permission" is from 1937. Green and red as signals on railways first attested 1883, as nighttime substitutes for semaphore flags. Green beret originally "British commando" is from 1949. Green room "room for actors when not on stage" is from 1701; presumably a well-known one was painted green.
Old English grenian (see green (n.,adj.)). Related: Greened; greening.