What does “красавчик” mean in Russian slang?

Красавчик literally means “handsome guy,” from красивый, “beautiful.” In casual speech it has become a shout of approval for something done well: get the answer right, score the goal, fix the bug, and a friend says красавчик!, “nice one!” It is about the deed, not looks, and it is friendly and informal, not something you say to your boss.

For a girl or woman, the usual praise for doing something well is умница!; красотка is mostly about looks. Красавчик sits alongside молодец (“well done”) but sounds more streetwise. The game here is a blindfolded juice test: each correct guess (сок яблочный) earns a красавчик, until the last “juice” turns out to be vodka.