Cockney rhyming slang - John o groat.

John O'Groats is considered as the northern tip of mainland Britain, Lands End being considered the most southern.

For the purposes of rhyming slang there is but one John O'Groat, unless you have several coats, yes John O'Groat is Cockney rhyming slang for coat.

Let's put it in a sentence: "My old man used to tell me to wear my john o'groat when it got nippy out", to translate; "My father used to tell me to wear my coat when it was a bit parky out".