also: ウェーイけい · うぇいけい · うぇーいけい
noun
rowdy young people; party-loving youth
Refers to a stereotype of young people, often college students, who are loud, energetic, and enjoy drinking and partying in groups. The term comes from the cheerful shout 'ウェーイ!'.
あのサークルはウェイ系が多いから、いつも飲み会がうるさい。
That club has a lot of rowdy party types, so their drinking parties are always noisy.
彼は見た目はチャラいけど、実はウェイ系じゃないんだよね。
He looks flashy, but he's actually not one of those loud party people.
From the interjection 'ウェーイ!' (a cheerful shout often used when toasting or celebrating) + 系 (type, group). The term emerged in Japanese youth slang to label a certain party-loving, loud social group.