The definition of gassed is slang for drunk or intoxicated, or for being out of energy. Being drunk from drinking two bottles of wine by yourself is an example of being gassed.

Is gassed a British slang?

gassed adjective (DRUNK)

What does calling someone gassed up mean?

It is a slang word but if someone is gassed up it means that they are made to think that they are better than they actually are.