I've always used this word, and likewise, have always wondered what it meant. Do you wanna know? You probably already do, but just in case you didn't, it means "OK. Tha's coo. Whatev. I'm down. Dope. Sure. Totally."
Turns out I've been using it correctly this whole time! That's a first for me, who in the past have used words like pontificate and execrable (which I would, of course, say in a French accent) microcosm (which I used completely opposite of it's definition) completely wrong or out of context.
Now, these are just a few examples, but you can imagine how excited I was to know that I was actually using a big word correctly. That's totally copacetic. Wait, is that redundant?