This is a quote from William Manchester's "A World Lit Only by Fire:
"Again and again, in recalling Satan's attacks on him, Luther uses the crude verb bescheissen, which describes what happens when someone soils you with his Scheiss. In another demonic stratagem, an apparition of the prince of darkness would humiliate the monk by "showing his arse" (Steiss). Fighting back, Luther adopted satanic tactics. He invited the devil to "kiss" or "lick" his Steiss, threatened to "throw him into my anus, where he belongs," to defecate "in his face" or, better yet, "in his pants" and then "hang them around his neck."
Is this at all accurate? I personally find it hilarious and truly hope that this actually occurred.
First, carefully pick up your copy of Manchester, walk slowly to the nearest bin, open the lid, and drop it in. Make sure that none of the dust on the pages contaminates you by scrubbing with strong soap.
Having said that, Luther wasn't averse to coarse language when talking about the devil so yes, while I haven't tracked down Manchester's quote, here's Heiko Oberman (reformation expert) on Luther:
I'd swear that somewhere there's a picture of Luther defecating in the pope's mouth but I've lost it somewhere in my files, but I'll reference this paper purely for the pictures and not for the content which I haven't read as an example of how crude things could get.