What does "Taking the piss" mean?
It means joking, teasing or making fun of someone rather than being serious. It can also mean behaving unreasonably or taking advantage of someone, depending on the situation.
The saying in a sentence
The saying
Taking the piss
Example
Are you taking the piss or is that genuine?
When you use it
Use it with mates who are comfortable with the language, either to check whether something is a joke or to call out an unreasonable request.
When you do not
It is coarse language and can sound confrontational. Avoid it in formal settings, with strangers or when teasing would hurt someone.