When two men have so much sex that the receiving partners cheeks get stuck together from the dried man juice like two piece of metal that have been welded. Also a manufacturing term.
Guy: Last night I was with Brian and I Butt Welded him.
Other Person: What is that?
Guy: I came really hard and after I pulled out, he didn't wash his cheeks properly so the next morning his butt crack was stuck shut.
Other Person: I am your Grandmother, why would you tell me about Butt Welding?