Green’s Dictionary of Slang

(orig. milit.) the arm.

1914		N.Z. Truth 31 Jan. 2/8: Do you get the twist and twirl [...] with a God forbid (a kid) on a false alarm (her arm).
1917		W. Muir Observations of Orderly 225: A man’s arm is his ‘false alarm’.
1919		Truth (Brisbane) 28 Sept. 2/2: I saw one Digger [...] with his twist and twirl, walking along the frog and toad with his false alarm round her brush and paste.
1942		Berrey & Van den Bark Amer. Thes. Sl.
1960, 1975		Wentworth & Flexner DAS.
1963		L.F. Freed Crime in S. Afr. 106: His ‘false alarm’ [is] his arm.
1982		G. Newbold Big Huey 248: false alarm (n) Arm.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 67/1: false alarm n. arm.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.