The punching of someone's asshole. The objective is not to actually enter the asshole, but usually as a retaliation when noticing that one's partner is about to defecate on one's face or chest during physical relations.
It can also be used in a more general sense when punching someone's exposed asshole for a myriad of reasons.
That dude was about to lay a Cleveland steamer
on me, so I gave him a chocolate shank.