Commonly used in Florida to describe sexual relations in which a man forcefully pulls down his partner's pants (similar to the tailgate of a pickup truck, hence the term) and then proceeds to violently assfuck them until they beg for mercy.
When performed properly, this technique is done without any lubrication, and the man must ejaculate in their partner's ass. Rookies often make the mistake of using lube, wearing a condom, or pulling out, which disqualifies this situation as a true "tailgate."
Lester: "Damn Tommy, everybody has been talking about you showing Amanda how to open the tailgate the other night after the party."
Thomas: "What the fuck? I'd never do that. We used a condom dumbass, so I didn't actually open her tailgate!"