Green’s Dictionary of Slang

buttfuck n.

[lit. and fig. uses of buttfuck v. (1)]

1. (also butt-bang) an act of anal intercourse.

[UK]R.O. Butler Alleys of Eden (1991) 147: Well-hung males looking for a fix or a butt fuck.
[UK]J. Morton Lowspeak.
[US]R.O. Scott Gay Sl. Dict. [Internet] anal intercourse: [...] Syn: butt-bang; butt-fuck.
[US]‘Bill E. Goodhead’ Nubile Treat [Internet] The butt-fuck that her mother had gotten was nice, and she was eager to receive the same thing from her father.

2. a tedious piece of work, i.e. pain in the arse n. (3)

[US]L. Heinemann Paco’s Story (1987) 129: Work’s work, but I tell you from the bottom of my heart that Iwo Jima was a sloppy, bloody butt-fuck.
[Aus]B. Moore Lex. of Cadet Lang. 68: butt fuck2 [noun] a severe, onerous, or thoroughly disagreeable task.

3. a disaster, a piece of victimization.

[Aus]J.J. DeCeglie Drawing Dead [ebook] And there it goes again, on and on, the bedlam of [...] the invariable buttfuck.

4. in attrib. use of sense 1.

[Aus]Picture (Sydney) 1 June 40/4: With all the hundreds of butt-fuck flicks around at the moment, it’s amazing porno starlets’ smoos haven’t shrivelled up through lack of use.