The person who, in the lead up to some social event, everyone gives their
money to so that they may purchase their
alcohol, even thought they only offered their
service to about
two people. That person is then burdened with bringing
copious amounts of grog to said event.
Particularly applicable to the one friend in a group of under-age people who has an older sibling or friend and therefore easy access to alcohol.
See also, Beer Sherpa