Abdomen, directly from the Latin abdomen, belly. Obscure origins, but most probably came from either of the latin words abdere (conceal) or adeps (fat)
The term was originally applied to the bellies of pregnant pigs, whose milk-laden mammary glands were eaten as delicacies in ancient Greece. Around 60 B.C., Cicero and the Greek comedy writers began using the word as a humorous insult to pot-bellied men. It was the Greek philosopher Pliny the Elder (23-79 A.D.) who first used the term in the anatomical sense familiar today.
Related usage: rectus abdominis. If well-defined, the rectus abdominis is colloquially called a "six-pack." (Note: often misspelled as “rectus abdominus”; a mnemonic for the correct spelling: i have a six-pack, u don't)
Pliny the Elder