Green’s Dictionary of Slang

rear end n.

(orig. US) the backside or buttocks.

[1787		‘Peter Pindar’ ‘Brother Peter to Brother Tom’ Works (1794) II 65: This watchful Lord, who boasts the knack, Behind His Sacred Majesty’s great back, Of placing for his latter end a chair].
1915		Labor Advocate (Cincinnati, OH) 18 Dec. 7/1: The Colobel [...] kept right on talking, with the result that union labor is ready to tie one of his own large tin cans to his rear end and set him adrift.
1929		C. McKay Banjo 185: I just ain’t gwine to have this rear-end facer trailing us.
1934		A. Halper Foundry 123: I ought to get my rear end kicked hard.
1943		H.A. Smith Life in a Putty Knife Factory (1948) 186: Muk’s [a chimpanzee] rear end is bare of foliage and gleams a fiery red its natural state.
1951		J.D. Salinger Catcher in the Rye (1958) 216: The other one, the brown one, was in his goddam cave and wouldn’t come out. All you could see was his rear end.
1961		R. Gover One Hundred Dollar Misunderstanding 27: Going up those narrow stairs, I almost bumped into her rear end.
1975		T. Berger Sneaky People (1980) 91: I don’t wanna to see you standing around with your finger up your rear end.
1977		Eve. News (London) 17 Aug. 1: He would ask us to give him shots in the rear end.
1988		in D. & C. Schneider Sound Off! 62: Even if it’s just a pat on the rear end. We don’t take any of it.
1998		A. Warner Sopranos 102: What about when it comes out of your rear end, honey?
1999		Observer 1 Aug. 28: I used to write fast – now my rear end gets tired.

In compounds

rear-end loader (n.) [pun on SE rear end]

(N.Z.) a male homosexual.

1988		McGill Dict. of Kiwi Sl. 92/1: rearend loader active male homosexual.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

In exclamations

my rear end!

a general excl. of disdain, dismissal, arrogant contempt.

1975

J. Wambaugh Choirboys (1976) 294: Booo! Zapata my rear end!