The adjective "game" in this phrase dates from the 18th century, and means "having the spirit of a game-cock" (a fighting cock), i.e. full of pluck, showing fight, spirited. So "Are you game" means "Are you up for it? Do you dare to do it?" (whatever "it" is).

Are you game meaning slang?

If someone asks you if 'you are game,' they want to know whether you are willing to try something; whether you are interested in doing something fun or unusual.

What does it mean when you call someone game?

to be ready for action; to be agreeable to participating in something. "I'm game," David replied when I suggested we go for a long hike.