I’ve always heard this as the first clown calling the second a dull ass. But I think now that this dull ass is a metaphorical ass – an analogy – not specifically the dull ass belonging to the second clown – but your dull ass as a generic kind of ass.
This is supported by his use of “thy brains” but the dull ass is “your dull ass” and the generic your is a speech pattern of the clown as in “your water” and “your whoreson dead body” – neither the water nor the dead body belong specifically to the man he’s talking to – but are a kind of colloquial way to describe a thing.
I mean, yes, he is likely comparing the second clown’s brains to a dull ass – but I think it might have less to do with how stupid he perceives the Second Clown to be and more to do with ceasing the efforts of trying to think. He has, after all, already liked his wits so he’s not entirely disrespectful of his fellow gravedigger’s intelligence. Anyways – this may be a dull ass that I’m cudgeling but I think this line is often why the Second gravedigger is played as the dumb one. It’s a facile choice based on the appearance of “dull ass” in a sentence.