Green’s Dictionary of Slang

the buttocks; often as on one’s beam-ends, lit. or fig. fallen over.

1827		T. Morton A School For Grown Children IV iii: Ha! ha! Brought him on his beam-ends.
1829		D. Jerrold Black-Ey’d Susan II i: I’ll overhaul him – I’ll bring him on his beam-ends.
1830		Marryat King’s Own II 94: Our first lieutenant was in his cot, on his beam ends with the rheumatiz.
1837		‘Nights At Sea’ in Bentley’s Misc. Dec. 617: I got fairly foozlified, and hove down on my beam-ends as fast asleep as a parish clerk at sarmon time.
1841		W.J. Neale Paul Periwinkle 509: I hope [...] to have every one of these fellows, from Alibi downwards, laid upon their beam-ends.
1843		T. Haliburton Sam Slick in England II 140: He’ll get off his beam ends, and right himself.
1865		‘The Scotch Fiddle’ in Rambler’s Flash Songster 35: She swears she cannot set upon her beam-end half a minute. / But figgets so upon her bum, there must be something in it.
1881		Trumble Sl. Dict. (1890).
1885		Bulletin (Sydney) 9 May 14/3: [A] horny-handed harvester got extensively ‘corned,’ and while struggling gloriously with the laws of gravity, he collided violently with the totalisator and very nearly shook that concern and the official in charge on their respective beam-ends.
1895		C. Crowe Aus. Sl. Dict. 7: Beam Ends, in a sitting position; also a person in difficulties is said to be on his beam-ends.