To be-dung, to shitten, to muck up, and many other synonyms. This is one of the rarer members of that set, which is a shame as it's the one of the nicest sounding. It was borrowed in 1635 from that lovely language French, from a verbalized form of merde, shit. French also has an antonym, démerder, which for some reason we did not pick up -- and you'd think it's be useful to say you're cleaning the crap off. Ah, well. For an example, here's one of the rare uses in the wild, by that lexical magpie masquerading as a poet, Robert Browning:
... a muckheap of a man,---L.
There, pillared by your prowess, he remains,