A Tac-Yack (Tactical Yack) is the act of expelling alcohol or food from the body via the mouth for tactical reasons. Such tactical reasons may include but are not limited to:
1. During Boat Races where you do not wish to get too drunk too quickly in order to win subsequent boat races.
2. During an all-you-can-eat buffet where you realize that you have eaten too much of the appetizers and cannot eat the delicious main course meals unless you empty your stomach.
Fred tac-yacked all over the floor of his ex-girlfriend's car. He then proceeded to break into her house and steal all of her jewelry.