Green’s Dictionary of Slang

sinker n.1

[sink v.]

1. (US Und.) one who cheats his partner(s) in crime by embezzling the takings of a scam.

[US]Matsell Vocabulum 80: […] sinkers Thieves who do not divide fair with their companions.

2. a drunkard [later use Aus.].

[Aus]G.H. Lawson Dict. of Aus. Words And Terms [Internet] SINKER — A drunkard.
[US]A.J. Pollock Und. Speaks n.p.: Sinker, a drunkard.

3. (S.Afr.) a judge.

[SA]L.F. Freed Crime in S. Afr. 106: A ‘sinker’, a ‘sticker’ or a ‘stretcher’ is a judge.