Green’s Dictionary of Slang

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

1859

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

2. a drunkard [later use Aus.].

1924		G.H. Lawson Dict. of Aus. Words And Terms 🌐 SINKER — A drunkard.
1935		A.J. Pollock Und. Speaks n.p.: Sinker, a drunkard.

3. (S.Afr.) a judge.

1963

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