I believe it's like get crazy or go crazy that's why some people say it before parties. Like: "yeah dude I'm going to get crunk at this party".

I thought it was just a way to say get drunk, without letting people know what your plans are. Mostly used by people under the legal drinking age lol. "a euphemism for getting really crazy and -blam!-ed up on marijuana and alcohol (stoned and drunk. Chronic plus Drunk = Crunk)."