Old English brod "brood, fetus, hatchling," from Proto-Germanic *brod (cognates: Middle Dutch broet, Old High German bruot, German Brut "brood"), literally "that which is hatched by heat," from *bro- "to warm, heat," from PIE *bhre- "burn, heat, incubate," from root *bhreue- "to boil, bubble, effervesce, burn" (see brew, v.).

"sit on eggs, hatch," mid-15c., from brood, n.. The figurative meaning ("to incubate in the mind") is first recorded 1570s, from notion of "nursing" one's anger, resentment, etc. Related: Brooded; brooding.