1640s, "hovering, overhanging" (as a mother bird does her nest), from present participle of brood (v.); meaning "that dwells moodily" first attested 1818 (in "Frankenstein").
"action of incubating," c.1400, verbal noun from brood (v.). Figuratively (of weather, etc.) from 1805; of mental fixations by 1873. Related: Broodingly.