Green’s Dictionary of Slang

(US) a container of fake (usu. chicken) blood, secreted on the body and used to simulate a gunshot wound.

1955

F. Kane ‘Return Engagement’ in Best of Manhunt (2019) [ebook] ‘Whose idea was the cackle bladder for shakedowns? [...] That gimmick was old when you were in rompers, baby. The old-time con men used it to cool down a mark who started to yell copper. The inside man would provide a gun loaded with blanks, the con man would have the thin rubber bladder filled with chicken blood between his teeth. When he bit the bladder it gave the effect of hemorrhaging from the mouth’.