Yesterday a friend asked me whether bellyache was considered rude or vulgar, and whether tummy ache or stomach ache were preferable in formal conversation. I thought that the word belly might be seen as vulgar and/or informal by some; that stomach ache might be better in formal situations, and that tummy ache tends to be used by and with children. Would you agree?
Belly comes from the Old English belg (bag, purse, bellows, pod, husk), from the Proto-Germanic*balgiz (bag), from the PIE base *bhelgh- (to swell), which is also the root of the Old Norse belgr (bag, bellows) and bylgja (billow); the Gothic balgs (wineskin), the Welsh bol (belly, paunch), the Irish bolg (abdomen, bulge, belly, hold, bloat), and the Latin bulga (leather sack). The English words bellows, billow, bolster, budget and bulge also come from the same root [source].
In English belly came to refer to the body during the 13th century, and the abdomen during the 14th century. By the late 16th century its meaning had been extended to cover the bulging part or
concave convex surface of anything. In the late 18th century some people in England decided that belly was vulgar and banished it from speech and writing – replacing it with stomach or abdomen. [source].