Green’s Dictionary of Slang

1. a Jew.

1927		(con. WW1) P. MacDonald Patrol 105: ‘’E may be a bleedin’ Buckle, but ’e can scrap’.
1930		(con. 1914–18) Brophy & Partridge Songs and Sl. of the British Soldier.
1960		J. Franklyn Dict. of Rhy. Sl. 44/2: buckle my shoe Jew. Early 20 C., and used in the army 1914–18. It is still occasionally used.
1969		S.T. Kendall Up the Frog.
1992		R. Puxley Cockney Rabbit.

2. (bingo, also buckle me) the number two.

1943		M. Harrison Reported Safe Arrival 85: ‘Two’ is [...] ‘Buckle me’ or ‘buckle’.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 18: Buckle my shoe Two.
2003		Guardian G2 8 May 4: ‘Thirty-two, buckle my shoe’ will give way to ‘Jimmy Choo’.