Green’s Dictionary of Slang

the buttocks.

1936		(con. 1830s–60s) ‘Miles Franklin’ All That Swagger 394: In my young days it was the bosom that was upholstered to the shape of a plum pudding [...] And to-day it is the b-t-m.
1948		J. Betjeman ‘Beside the Seaside’ Sel. Poems [poem] No friendly wallop on the B.T.M. / No loving arm-squeeze and no special look.
2002		(ref. to 1940s) B. Morrison Things My Mother Never Told Me 145: Now the last and most vicious kind of VD, syphilis [...] It can take as long as three months before the sore appears on your penis – that’s supposing you’re not a pansy, who’s more likely to get his on his b-t-m.
2024		J. Meades Empty Wigs (t/s) 337: ‘Naughty boy. You could get your bottom smacked. Your BTM-ee-kins’.