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late 14c. (early 14c. in Anglo-French), from Old French grané (with -n- misread for -u- -- the character used for -v- in medial positions in words in medieval manuscripts) "sauce, stew," probably originally "properly grained, seasoned," from Latin granum "grain, seed" (see corn (n.1)). See discussion in OED. Meaning "money easily acquired" first attested 1910; gravy train (1927) was originally railroad slang for a short haul that paid well.
Money or other valuables beyond what one actually earns or needs; a bonus or excess: Once we make back our expenses, everything else is gravy (1910+)