Green’s Dictionary of Slang

croaker n.3

also croker
[croak v.2 (1)]

1. (UK Und.) a hanging.

[UK]Worcester Herald 26 Dec. 4/3: A croaker, a hanging.

2. one who has collapsed, i.e. is metaphorically ‘dead’.

[UK]Leaves from Diary of Celebrated Burglar 22/2: Jack had it [i.e. a gin-flask] last [...] but I suppose he is a ‘croaker’ by this time, and if you ain’t d—d quick, some of them French b—rs will ‘shaver’ him before you get there.

3. a person who is dying, who cannot be saved, a corpse.

[UK]Metropolitan Mag. XIV Sept. 334: ob and I was riding alongside the foremost boy, bouncing to make him a croaker if he did not pull up.
[UK] ‘The Saint Giles’s Jade’ in Nancy Dawson’s Cabinet of Songs 45: He vos nearly a croker with fright.
[UK]A. Mayhew Paved with Gold 353: Here’s the boy, pretty near a croaker.
[UK]Leaves from Diary of Celebrated Burglar 145/1: A nuther blarsted step I woiynt do fur’m, iv they sall kill mi, an’ I’m ne’er a kroaker neow.
[UK]Wild Boys of London I 140/2: ‘’Pon my sivy,’ he said, ‘I thought I was a clean croaker.’.
[UK]Sl. Dict. 133: Croaker a dying person beyond hope; a corpse. The latter is generally called a ‘stiff’un.’.
[UK]Manchester Courier 24 Mar. 9/7: When the dog were found a croker [sic] in the morning they swore it was me.
[UK]Farmer & Henley Sl. and Its Analogues.

4. (orig. US black) a murderer.

[UK]J. Morton Lowspeak 45: Croaker – a professional killer.
[US](con. 1940s–60s) Décharné Straight from the Fridge Dad 39: Croaker [...] 2. Murderer, ie: one who croaks someone.

In derivatives

croakery (n.)

(US) murder.

[US]R.L. Bellem ‘Focus on Death’ Hollywood Detective Jan. [Internet] Is that all it means to you? Don’t you realize this is croakery?