tail-gat’ing (v):

1) attending a social gathering in which food and drinks are served at or near the back end of a parked vehicle that usually occurs in a parking lot before or after a public event

2) the activity of cooking food and having drinks out of the back of your car in the parking lot before going in to a public event such as a sports games, concerts, races and other live events

tail-gate party (n):

a social gathering or picnic at which an informal meal is served, typically from the back of a parked vehicle, usually in the parking lot of a sports stadium