Green’s Dictionary of Slang

(US) a .32 calibre pistol.

1927		B. Cormack Racket Act I: Stealing a Rolls [...] And carrying one of those boodwah thirty-twos!
1932		J. Dixon Free To Love 182: It’s a .32 with a silencer.
1969		D. Pendleton Executioner (1973) 15: Bolan’s .32 was in his hand, but it seemed small comfort in the face of the burpgun that was methodically spraying the area about him.
1978		S. King Stand (1990) 298: I believe it’s a .32.
2000		C. Cook Robbers (2001) 224: Only he carried a thirtytwo, not a twentytwo.