Green’s Dictionary of Slang

(US) a synon. for motherfuck n. (1)

1980		W. Kotzwinkle Jack in the Box 142: You mudfucks comin’?
2002		S. Jewinski MooMoo Fever 11 Jan. 🌐 I could have sworn that waking up promised me at least a fleeting glimpse at the world outside, but, damn my eyes, I was wrong as fuck. Wrong as mudfuck.