muy caja

Colombian slang, specifically from Medellin, it means that something is really funny, it always goes with the adverb "muy" before "caja", but sometimes you might find it as "tan caja" when describing that someone is being really funny right now.

Someone says a joke.
Me: Ese man es muy caja! // That guy is so funny!

Alternatively.
Me: Que man tan caja! // Such a funny dude!

My friend Deyker trips on the street in front of everybody, humiliating himself and making me laugh so hard.
Tomorrow at school:
-La caida de Deyker ayer fue muy caja!