Cut the Crap
What it means:
No…It does not mean to literally cut poo! But it is an informal idiom and can be quite impolite.
When you tell someone to cut the crap it actually means that you are asking them to stop lying (as a joke or seriously) or to go straight to the point.
When to use it:
Definitely NOT during an exam or to your boss.
Use this expression when someone is trying to avoid giving you the information you want, when someone is very obviously lying to you or when you want to avoid the unnecessary details of a story.
You should be quite upset, irritated or even angered when using this expression. But remember, this is extremely informal and can be rude. Don’t use this idiom with everybody. But a friend, for example, should be OK with you saying that, if done in a friendly/joky way.