The beer left in the pitcher after 3 pints have been poured is deemed buyer's right. That is, they can lay claim to the remaining beer.
Trevor claimed buyer's right and finished up the the pitcher
if someone wants you to buy either fags or alcohol but are too young you have buyers rights which means you take 10% of the amount that you bought and keep it for yourself or give it to someone else.
for example if you buy a 14 year old a pack of 10 mayfair...then you have get buyers rights which gives you the right to keep one and smoke it yourself or if you buy someone a crate of 20 stella you get to keep 2