Green’s Dictionary of Slang

croaker n.3

also croker
[croak v.2 (1)]

1. [mid-19C] (UK Und.) a hanging.

2. [mid-19C] one who has collapsed, i.e. is metaphorically ‘dead’.

3. [mid–late 19C] a person who is dying, who cannot be saved, a corpse.

4. [1980s+] (orig. US black) a murderer.

In derivatives

croakery (n.)

[1940s] (US) murder.