It is the area of a man's body located between the balls and the anus. Likened to the chode, or taint; and more affectionately referred to as "the gooch". This area becomes a baenus when it is protruding. Such protrusions can result from lack of sack, or the overall inability to maintain any semblance of testicular fortitude.
His baenus is so swollen because his woman won't let him out of the house....and he won't tell her otherwise.