When a man and a woman are having sex and the guy pulls out and cums either on her back, her stomach, or his own stomach. Then, in an attempt to keep said cum off of the bed, the individual with the cum on them, puts their arms against their own sides to catch the cum from running off of them onto the sheets. This area between your side and your arm is the "gutter".

When some little bitchboy is acting like a little tiny pee pee poo poo baby and messing up the group plans.