A "friend sip" is the absolute limit of liquid a friend is allowed to consume from your drink when you are polite enough to share (one mouthful). No more than a "Friend Sip" can be taken by a friend, preferably less.

when you're hungover as shit after a spending a night drunk as a skunk

When two or more people get together to drink; a drinking party; any type of beverage is included: coffee, tea, soda, alcohol.