Pregaming is the informal, mostly American habit of drinking alcohol at home — or at a friend’s house — before you go out to a party, a club, a bar, or a sports match. People do it to get in the mood, to spend less on expensive drinks outside, or both. The related verb is to pregame. It is young-adult and student talk; you will not see it on a formal invitation. Though actually, the “game” here is not a video game: it started around drinking before a real sports game, then spread to any night out. In Taiwan, friends often do the same thing without this English name — they just say they will drink a bit first at someone’s place. The word can sound casual or a bit reckless, depending who says it. It names the practice; a single drink at home is not automatically “pregaming” unless that is the plan before you go out.