Figuratively, "the smallest possible quantity," from late 14c. From early 15c. in English as the smallest unit of weight (originally the weight of a plump, dry grain of wheat or barley from the middle of the ear). From late 14c as "roughness of surface; a roughness as of grains." In reference to wood, "quality due to the character or arrangement of its fibers," 1560s; hence, against the grain (1650), a metaphor from carpentry: cutting across the fibers of the wood is more difficult than cutting along them.
Earliest sense of the word in English was "scarlet dye made from insects" (early 13c.), a sense also in the Old French collateral form graine; see kermes for the evolution of this sense, which was frequent in Middle English; also compare engrain. In Middle English grain also could mean "seed of flowers; pip of an apple, grape, etc.; a berry, legume, nut." Grain alcohol attested by 1854.
It forms all or part of: corn (n.1); filigree; garner; garnet; grain; granary; grange; granger; granite; granular; granule; grenade; grenadine; kernel; pomegranate.
It is the hypothetical source of/evidence for its existence is provided by: Latin granum "seed," Old Church Slavonic zruno "grain," Lithuanian žirnis "pea," Old English corn.
late 14c. (early 14c. in Anglo-French), from Old French grave, graue, apparently a misspelling of grané "sauce, stew," with -n- misread for -u- — the character used for -v- in medial positions in words in medieval manuscripts. The French word probably originally meant "properly grained, seasoned," from Latin granum "grain, seed" (see grain (n.)).
The meaning "money easily acquired" is attested by 1910; gravy train (by 1899) as something lucrative or productive is said to have been originally railroad slang for a short haul that paid well. Gravy-boat "small, deep dish for holding gravy or sauce" is from 1827.