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
Well Ori's my grog bunny so I just gave him $20