When a man ejaculates during intercourse, then feels shame immediately after the euphoria of climaxing.
"Yea, I picked this girl up last night...I Shummed so hard."
When a person gets to the max compacity of their shit and sperm to the point it causes pain then releases both the shit and the cum at once to feel the maximum relief known to man. Shum
Or
When you feel pleasure and intoxication at the same time but cannot which is more important so you decide to do it at the same time.
Friend: stfu before I kiss you baby girl
You: be right back I gotta shum real quick