Taking the piss

It can just mean making fun of someone or something, but it can also mean taking liberties, like hogging all of something meant to be a shared resource for yourself.

That's the 35th time you've been back to the all-you-can-eat buffet table and filled three plates, Dave. You are taking the piss now.

taking the piss

It can just mean making fun of someone or something; it can mean blatantly flouting what one knows to be a rule or normally acceptable behaviour, just because one can get away with it; it can mean selfishly taking advantage of someone, or of a situation, or taking a liberty, especially one involving hogging all of what is understood to be a shared resource for oneself.

That's the eighth time you've been back to that all-you-can-eat buffet table and filled three plates, Dave. You're taking the piss now.