1832, "grass yielding edible grain and cultivated for food," originally an adjective (1818) "having to do with edible grain," from French céréale (16c., "of Ceres;" 18c. in grain sense), from Latin Cerealis "of grain," originally "of Ceres," from Ceres, Italic goddess of agriculture, from PIE *ker-es-, from root *ker- (2) "to grow." The application to breakfast food cereal made from grain is American English, 1899.
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]
"Indian millet," 1590s, from Modern Latin Sorghum, the genus name, from Italian sorgo "a tall cereal grass," probably from Medieval Latin surgum, suricum (12c.), perhaps a variant of Latin syricum "Syrian," as in Syricum (gramen) "(grass) of Syria," from Syria, a possible source of the plant or its grain in ancient times.
hybrid cereal grass, 1952, from Modern Latin Triti(cum) "wheat" (literally "grain for threshing," from tritus, past participle of terere "to rub, thresh, grind," from PIE root *tere- (1) "to rub, turn") + (Se)cale "rye."
1550s, "the grain of Indian corn;" 1580s of the cereal plant of the grass family that produces it, from Cuban Spanish maiz, from Arawakan (Haiti) mahiz, the native name of the plant. In Europe it was formerly also called Turkey corn; like the fowl, this is from mistaken notions of its origin.