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
When a male ejaculates into another person’s anus, and the receiver goes to the toilet & out comes the shum. The mix of the sperm & poo isn’t ‘cum’, it’s ‘Shum’. Shit and cum.
‘Oh wow! I just been to the loo after he fucked me in the arse, and SO much shum poured out!’