Green’s Dictionary of Slang

backside n.

[backside has been SE in the UK since 1500 but is still considered improper in the W.I.]

[late 18C+] the posterior, the buttocks, esp. as synon. with arse n. (1), as in kiss one’s backside, kick one’s backside etc.

In derivatives

backsiding (n.)

[1990s+] (W.I.) a severe thrashing.

In compounds

backside artist (n.) [-artist sfx]

[1980s+] (US gay) a male homosexual who practises anal intercourse.

In phrases

backside of nowhere (n.)

[1960s+] (US) nowhere, a very out-of-the-way place.

In exclamations

my backside!

[late 18C+] a general excl. of disdain, dismissal, arrogant contempt.