Old English græs, gærs "herb, plant, grass," from Proto-Germanic *grasan (source also of Old Frisian gers "grass, turf, kind of grass," Old Norse, Old Saxon, Dutch, Old High German, German, Gothic gras, Swedish gräs"grass"), which, according to Watkins, is from PIE *ghros- "young shoot, sprout," from root *ghre- "to grow, become green," thus related to grow and green, but not to Latin grāmen "grass, plant, herb." But Boutkan considers grāmen the only reliable cognate and proposes a substrate origin.
As a color name (especially grass-green, Old English græsgrene) by c. 1300. Sense of "marijuana" is recorded by 1932, American English. The grass skirt worn by people native to tropical regions is mentioned by 1874; the warning to keep off the grass by 1843 (in New York City's Central Park). Grass-fed of cattle, etc., (opposed to stall-fed) is from 1774.
1520s, the earliest recorded sense is "mistress;" the allusion to grass is not clear, but it commonly was believed to refer to casual bedding (compare bastard and German Strohwitwe, literally "straw-widow," and compare the expression give (a woman) a grass gown "roll her playfully on the grass" (1580s), also euphemistic for the loss of virginity). Revived late 18c. as "one that pretends to have been married, but never was, yet has children;" in early 19c. use it could mean "married woman whose husband is absent" (and often presumed, but not certainly known to be, dead), also often applied to a divorced or discarded wife or an unmarried woman who has had a child. Both euphemistic and suggestive.
[G]rasse wydowes ... be yet as seuerall as a barbours chayre and neuer take but one at onys. [More, 1528]
GRASS WIDOW, s. a forsaken fair one, whose nuptials, not celebrated in a church, were consummated, in all pastoral simplicity, on the green turf. [Rev. Robert Forby, "Vocabulary of East Anglia," London, 1830]
1580s (earlier buffel, 1510s, from French), from Portuguese bufalo "water buffalo," from Medieval Latin bufalus, variant of Latin bubalus "wild ox," from Greek boubalos "buffalo," originally the name of a kind of African antelope, later used of a type of domesticated ox in southern Asia and the Mediterranean lands, a word of uncertain origin. It appears to contain bous "ox, cow" (from PIE root *gwou- "ox, bull, cow"), but this is perhaps a folk-etymology association.
Wrongly applied since 1630s to the American bison. The other Germanic words (Dutch buffel, German Büffel, Danish böffel, etc.) are from French; from Medieval Latin come Russian buivolu, Polish bujwoł, Bulgarian bivol, etc. Buffalo gnat is recorded from 1822. Buffalo chip "dung of the American bison," used for fuel on the U.S. plains, is from 1840.