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.
"meatless, made without flesh; abstaining from flesh," 1680s, from French maigre "lean, spare, meager," as a noun, "lean meat, food other than meat or gravy" (see meager).
"to soak (cooking meat) in gravy or molten fat, moisten," late 14c., of unknown origin, possibly from Old French basser "to moisten, soak," from bassin "basin" (see basin). Related: Basted; basting.
1807, American English, "sauce, gravy; any thick liquid," from Dutch doop "thick dipping sauce," from doopen "to dip" (see dip (v.)). Used generally by late 19c. for any mixture or preparation of unknown ingredients.
Extension to "narcotic drug" is by 1889, from practice of smoking semi-liquid opium preparation. Meaning "foolish, stupid person" is older than this (1851) and may be from the notion of "thick-headed," later associated with the idea of "stupefied by narcotics."
Sense of "inside information" (1901) may come from knowing before the race which horse had been drugged to influence performance (to dope (v.) in this sense is attested by 1900). Dope-fiend is attested from 1896, "a victim of the opium habit."