To shit and cum at the same time. This usually occurs when the victim is listening to beautiful music or is overcome by intense feelings of pleasure and emotion. Another occurrence is when you're jerking off and taking a shit on the toilet at the same time.
I shummed myself at the pink floyd concert when the comfortably numb guitar solo started
A Shum is a person who buys new versions of phones as soon as they come out and disregard their perfectly fine old versions of the phone.... Usually people in their late teens and 20's, who want to look "cool" by having the newest versions of phones)
That Shum Dave just spent $300.00 on the new Galaxy phone that came out yesterday, although his old Galaxy still works just fine.