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; cits. c.1698, 1785, 1796, 1811, with caffan, are misprints, due to confusion with the ‘long s’; Hotten (1859) notes that this has been ‘ridiculously inserted’]

(UK Und.) cheese.

[UK]Harman Caveat for Common Cursetours in Viles & Furnivall (1907) 83: cassan, cheese.
[UK]Groundworke of Conny-catching A3: She hath a Cackling Chete, a gruntinge chete, ruffe peck, cassan.
[UK]Dekker Lanthorne and Candle-Light Ch. 1: Cassan is Cheese, and is a word barbarously coynde out of the substantive Caseus which also signifies Cheese.
[UK]J. Taylor Crabtree Lectures 191: Mort. Ile tell thee queere Cove, thou must maund at the Gigger for Pannum and Casum, or a cheat of queere bowse, or Kacklen Cheate, and whid rumpsie.
[UK]R. Brome Jovial Crew II i: Here’s Ruffpeck and Casson, and all of the best, / And Scraps of the Dainties of Gentry Cofe’s Feast.
[UK]Head Eng. Rogue I 47: Cosan, Caseus, Cheese. [Ibid.] 48: Cassan, Cheese.
[UK]Head Canting Academy (2nd edn).
[UK]R. Holme Academy of Armory Ch. iii item 68c: Canting Terms used by Beggars, Vagabonds, Cheaters, Cripples and Bedlams. [...] Cassan, Cheese.
[UK]B.E. Dict. Canting Crew n.p.: Caffan, c. Cheese.
[UK]J. Hall Memoirs (1714) 11: Casum, Cheese.
[UK]A. Smith Lives of Most Notorious Highway-men, etc. (1926) 203: Cassin, cheese.
[UK]New Canting Dict.
[UK]Bailey Universal Etym. Eng. Dict.
[UK]Canting Academy, or the Pedlar’s-French Dict. 111: Bread and Cheese, Pannum & Causum.
[UK]Grose Classical Dict. of the Vulgar Tongue n.p.: Caffan. (cant) cheese.
[UK] ‘Flash Lang.’ in Confessions of Thomas Mount 19: Cheese, caz.
[UK]G. Andrewes Dict. Sl. and Cant.
[UK]Lex. Balatronicum [as cit. 1785].
[US]Commercial Advertiser (N.Y.) 1 Feb. 2/3: After roystering at the Theatre, they broomed to a neighboring bousing ken [...] one told the landlord to flick him some panea and cassan, [...] while the others commenced smashing the flickers and glims.
[UK]G. Kent Modern Flash Dict. 9: Cass – cheese. [Ibid.] 13: Flick me some panea and cassan – cut me some bread and cheese.
[UK]Flash Dict. in Sinks of London Laid Open.
[UK]Hotten Dict. of Modern Sl. etc.
[UK]Hotten Sl. Dict. 95: CASSAM, cheese.
[UK]B.M. Carew Life and Adventures.
[US]Trumble Sl. Dict. (1890) 10: Cass: cheese.

In phrases

as good as caz

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

[UK]Vaux Vocab. of the Flash Lang. in McLachlan (1964) 231: As good as caz, is a phrase signifying that any projected fraud or robbery may be easily and certainly accomplished; any person who is the object of such attempt and is known to be an easy dupe, is declared to be as good as caz, meaning that success is certain.