To get on the piss, or getting on the piss, refers to the act of drinking alcoholic beverages, most typically with friends, and getting heavily intoxicated as a result.
It's is a British term that people often use to say 'are you joking' or 'are you serious'. 'Piss' means 'urine'. Someone who takes the piss a lot is often called a 'piss take' since they 'take the piss'.