Green’s Dictionary of Slang

tartan n.

[? rhy. sl.]

(UK Und.) cocaine.

[UK]K. Sampson Outlaws (ms.) 62: It’s times like this I wish I was into the tartan. [...] She’s properly a tartan fiend, this one. That’s your password with a Judy like this — cocaine.
[UK]K. Sampson Killing Pool 45: I’ve been intimately involved in the seizure of over two hundred tonnes of class-A tartan.