"Crafty Brown" is a term used to describe just about anything that is done in a crafty way, mainly in the context of school life. Originally meaning "a quick sneaky fag" (probably at school), the term developed to mean any number of things of a "crafty" nature, including but not limited to: masturbation (in or out of school), copying homework, lying, sleeping during lessons, smoking, taking drugs, drinking alcohol, taking a quick poo (this is crafty because no-one is EVER supposed to take a dump at school), skiving off lessons and skiving off school.
It can also mean a person of asian descent who is crafty or shady in nature.
"Hey, im just goin' for a crafty brown" (means what you want it to mean)
"That Kiran is a right crafty brown"