Green’s Dictionary of Slang

butt adv.

[butt n.1 (1a), seen as something physically powerful]

[1980s+] (US campus) a general intensifier, very, very much, extremely, incredibly, truly.

In compounds

butt-ass (adj.) [-ass sfx]

[1980s+] (US) very.

butt-crazy (adj.)

[1990s+] (US teen) utterly, completely insane.

butt-ugly (adj.) (also bugly, butt, buttly) [butt n.1 (1a) + SE ugly] [1980s+] (US)

1. very ugly.

2. a general derog.