A person who is alot older than they look. Named after the 80's actor Ralph Macchio, because he was 22 years old when he starred as the karate kid and played a 16 year old.
Dude 1: Hey, check out that punk teenager.
Dude 2: He's not a teenager, Dude 1. He's like 23 years old.
Dude 1: Really?! Man.... what a Macchio!