(noun)
in American usage, an informal social gathering with food and drinks, held at/near the back of a parked vehicle like a pickup truck, usually in the parking lot of a sports stadium before a game;
e.g. They are going to a tailgate before the game on Sunday.
(verb)
in informal usage, to drive extremely close behind a vehicle, typically in a dangerous way;
e.g. We grew worried when a black car began to tailgate us on our way home.
(verb)
in informal American usage, to host or attend an informal social gathering with food and drinks served at/near the back of a parked vehicle, usually in the parking lot of a sports stadium before a game;
e.g. We usually like to tailgate before going to watch a football game.