the.com/gravy

means A sauce made from the juices and fat of cooked meat, thickened and seasoned, and by extension any easy, unearned extra on top of what you already have.

from Gravy comes from Old French 'grané,' a spiced sauce or broth, from 'grain' meaning spice. The popular story is that it's a medieval scribal error — someone misread 'grané' as 'gravé' and English inherited the typo — but this is unconfirmed folklore, not settled fact. The slang sense of 'easy extra money or benefit' is a much later American development, first showing up in the early 1900s.