Old English belg, bylig (West Saxon), bælg (Anglian) "leather bag, purse, bellows," from Proto-Germanic *balgiz "bag" (cognates: Old Norse belgr "bag, bellows," bylgja "billow," Gothic balgs "wineskin"), from PIE *bholgh-, from root *bhelgh- "to swell," an extension of *bhel- (2) "to blow, swell" (see bole). Meaning shifted to "abdomen of a human or animal" (late 13c.) as the old plural form of the noun emerged as a separate word (see bellows). Meaning "bulging part or concave surface of anything" is 1590s. The West Germanic root had a figurative or extended sense of "anger, arrogance" (as in Old English bolgenmod "enraged;" belgan (v.) "to become angry"), probably from the notion of "swelling."
Indo-European languages commonly use the same word for both the external belly and the internal (stomach, womb, etc.), but the distinction of external and internal is somewhat present in English belly/stomach; Greek gastr- (see gastric) in classical language denoted the paunch or belly, while modern science uses it only in reference to the stomach as an organ.
As a personal name from 12c. From c. 1200 as a symbol of gluttony. Belly-naked in Middle English was "stripped to the belly, completely naked." Fastidious avoidance of belly in speech and writing (compensated for by stretching the senses of imported stomach and abdomen, baby-talk tummy and misappropriated midriff) began late 18c. and the word was banished from Bibles in many early 19c. editions. Belly-punch (n.) is attested from 1811.
"to swell out," 1620s, from belly (n.). Related: Bellied; bellying. Old English belgan meant "to be or become angry" (a figurative sense). A comparable Greek verb-from-noun, gastrizein, meant "to hit (someone) in the belly."