A real S.O.B., a step up from motherfucker (but perhaps less offensive when heard in public). Evolved from motherfucker and inspired by the monstrocity Mothra, a foe of the famous Godzilla.
"That mothrafucker just stepped on my sac!"
"Mothrafucking Godzilla is always going around stepping on people's sacs."
"Only a real mothrafucking mothrafucker would commit such an inconsiderate act."