Slang word most often used by Britain, New Zealand and Australia.
American teenagers use it because they just follow other countries, thinking it makes them Indie or something.
American: Hey Mate.
British: No you can't use that, stop trying to be cool and stuff, it just fails, be an American and say something like 'Hey fatty'.
Australian: Yeah, people to we try to copy you, but you copy us, stop trying to use mate.
New Zealand: Finally people understand ...