X-pac, are you in there? was said by Adam as he looked into a car window one day. He and Liam were walking down the street, when he randomly put his head up to a window and said "X-pac, are you in there?" X-Pac is a famous WWF wrestler that Adam was a big fan of.
Liam later replicated this and made it a catchphrase by using his tape recorder and recording Jay react to him yelling it really loudly.