fartmanteau
Derived from the word portmanteau, which is when 2 different words are joined to create another word (e.g. fantabulous, which is the combination of fantastic & fabulous), fartmanteau is when the made-up word falls under toilet humor .
Dude 1: Duuuuude, I thought up another word today while smoking out. Shitarded. It's when something is both shitty & retarded.
Dude 2: That's dope.
Dude 1: No...That's a fartmanteau, my friend.
Dude 2: That's dope.
Dude 1: No...That's a fartmanteau, my friend.