"A style of artistic expression characteristic of a particular individual, school, period, or medium: the idiom of the French impressionists; the punk rock idiom."
Therefore can be used as the perfect excuse to get out of doing stuff while still leaving the impression that you are an intelligent, intellectual individual
Gaz: Nah, it's against my idiom
John: Huh, yeah... (shuffles away)
Gaz is now free to see John's mum, knowing John will be baffled for the next 48 hours