Old English græs, gærs "herb, plant, grass," from Proto-Germanic grasan (cf. Old Norse, Old Saxon, Dutch, Old High German, German, Gothic gras, Swedish gräs), from PIE *ghros- "young shoot, sprout," from root *ghre- "to grow, become green" (related to grow and green).
As a color name (especially grass-green, Old English græsgrene) by c.1300. Sense of "marijuana" is first recorded 1938, American English. Hawaiian grass skirt attested from 1937; keep off the grass by 1850.