It has not been discussed why the movie got the title "Belly", but looking at the storyline, it's pretty much getting into the idea of being in the "belly of the beast".
"Belly of the beast" means being in the middle of a very bad situation or a dangerous place.
Sources also say that "belly" is a street term for why people always risking their lives for the money. People need to feed their bellies to stay alive and to survive.
"Belly" Film Synopsis
Ever since they were kids, Sincere (Nas) and Buns (DMX) have lived life close to the edge, doing whatever it takes to survive. As adults, they build up their kingdom of crime on drug dealing and robbery. But Sincere grows weary of the criminal lifestyle and joins a black Muslim religious group. Buns, on the other hand, sinks deeper into criminality and faces serious prison time. The cops offer him a deal, however -- assassinate the head of the Muslim group, and he will go free.
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