Green’s Dictionary of Slang

bilk n.

[cribbage jargon balk, to spoil an adversary’s score in their crib ]

1. [mid-17C–mid-18C] an empty, meaningless statement.

2. [late 17C–mid-18C] a hoax; an act of cheating.

3. [late 18C+] (US Und.) a swindler or cheat.

4. [mid-19C] (US) a disappointment.

5. [1940s] (US) a form of swindle worked on a brothel madam.

In phrases

on the bilk

[mid-19C] cheating, swindling.