1510s, "apt or fit to breed," from brood (v.) + -y (2). Figuratively, of persons, "inclined to think long and deeply," from 1851. Also, in modern use (by 1980s), sometimes "full of maternal yearning." Related: Broodily; broodiness.
mid-15c., "sit on eggs for the purpose of hatching them," from brood (n.). The figurative meaning "meditate long and anxiously" (to "incubate in the mind") is first recorded 1570s, from notion of "nursing" one's anger, resentment, etc. Related: Brooded; brooding. Brood mare "female horse kept for breeding" is from 1829.
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy).