(n) a person that accompanies his friends to a party and tries to consolidate them, keep them out of trouble, holds back girls' hair while they puke, and makes sure they get home safely. Sheepdogs are not necessarily sober, but are distinguished by their genuine concern for their friends, never taking advantage of a girl when she is wasted or leaving.
Drunk Girl: I'm wasted are you wasted?
Sheepdog: I'm tipsy, but I gotta be the sheepdog tonight and make sure my friends get home okay.
Drunk Girl: Aww...