belly (n.) Look up belly at Dictionary.com
Old English belg, bylg (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 "body" (late 13c.), then focused to "abdomen" (mid-14c.). 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").

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. 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.
belly (v.) Look up belly at Dictionary.com
"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."