A great resource for correct grammar and style in writing.

IDIOM: An idiom is just a way of expressing something that has been sanctified by use — often in violation of apparent logic, or at least not having an obvious logic behind it.

Some idioms can sometimes be figured out from the component words: if I say something is a dime a dozen, you can probably figure out that it means "cheap" or "common." Others are impossible to figure out logically. If you get on my nerves and I tell you to piss off, I'm asking you to go away — not to urinate from a great height.