Green’s Dictionary of Slang

bumface n.

[bum n.1 (1) + -face sfx]

a general term of disdain, usu. only used by children.

[US]L.A. Times 24 May 11/7: ‘When Bumface Schneider came up they pasased him along to the morgue’.
[UK]K. Amis letter 12 Jan. in Leader (2000) 155: I had an impudent letter from Caton (perhaps I told you) asking me to call on him and ‘report sales’ the BUMFACE the bumface.
[SA]P. Slabolepszy ‘Boo to the Moon’ in Mooi Street (1994) 114: How’s ou bum-face?
[Aus]T. Winton That Eye, The Sky 34: Piss off, bumface.
[UK]Guardian 4 July 51/1: ‘In your dreams, bum-face’.
[UK]Guardian Education 13 Mar. 44/1: ‘Is that you Bumface [...] You’re still the same bossy Bumface’.