person2: actually, my trousers are just a bit short
person1's friend, person3: yeah thats what jack-ups are, you idiot
person2: commonly thought of as meaning trousers that are too short, 'jack-ups' are actually trousers that, surprise surprise, have been 'jacked' up. That means that the waistband has been pulled higher than necessary, possibly up to the naval (bellybutton) or chest.
person1 & person3: youre such a nerd