A cackle-bladder or cackle bladder is a rubber bladder filled with fake blood, used to fake a person's death.
Its name is derived from the fact that traditionally chicken blood was used.
As part of a con, a cackle-bladder can be used to fool the mark (victim) into believing that one of the con artists has been killed in order to avoid them seeking retribution.
[1] In espionage, a cackle-bladder can be used as a basis for blackmail.
[2] This method has been popularised through the BBC television series Hustle.