Green’s Dictionary of Slang

cassan n.

also cass, cassam, cassin, casson, casum, caz, cosan
[Rom. cas, cheese; cf. SE casein, the milk ingredient that is the basis of cheese]

[mid-16C–mid-19C] (UK Und.) cheese.

In phrases

as good as caz

[early 19C] (UK Und.) easy, simple, referring to any projected fraud or robbery, or a person who is to be made a victim of either.