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".
Chris: I am about to cum.
Jenna: Well I just washed my sheets so be careful where you cum.
Chris: Then put up your gutters.
Jenna: Well I just washed my sheets so be careful where you cum.
Chris: Then put up your gutters.
by modena April 19, 2008