Green’s Dictionary of Slang

1. synon. for motherfucker n. (1)

1966		in H.S. Thompson Hell’s Angels (1967) 40: The ‘Mofo’ Club from San Francisco.
1989		(con. 1968) W.E. Merritt Where the Rivers Ran Backward 20: Get your white ass in the truck, mofo.
1989		H. Rawson Dict. of Invective (1991) 258: Such variations on the unstated theme as M.F., Marshall Field, and Mister Franklin, the slurred mo’-fo’ and muh-fuh, the rococo triple-clutcher, popularized by black truck drivers in army construction battalions; and the clipped momma, mother, and muther.
1997		Eble Campus Sl. Fall 5: mo-foe – an abbreviation for ‘mother-fucker’ [...] ‘He’s one crazy Mo-Foe.’.
2000		P. Beatty Tuff 107: ‘Whose theory is that?’ ‘It’s my theory, mofo,’ Gusto answered.
2014		Pando Qly Spring 52/1: Did a year in Najran, mofo, and that’s like a year in Pelican Bay.
2019		Twitter 21 Mar. 🌐 864637 mofos have signed the petition. Don't click the link to the petition unless you haven't signed it!
2020		S.A. Crosby Blacktop Wasteland 89: ‘You is one driving mofo’.

2. synon. for motherfucker n. (4)

1989		W.E. Merritt Where the Rivers Ran Backward 20: ‘Shee-it, mofo. You mofo lucky they sent me along.’ [...] It wasn’t till I saw Green the next morning that I knew we were still friends.
1997–2001		Online Sl. Dict. 🌐 mo fo n 1. good friend; BUDDY. Abbreviation of ‘mother fucker’. (‘That’s my mo fo over there.’).