Chookas!

I was at work last night (we had our first preview) and everyone was running around with the normal ritual of saying "chookas".

It suddenly occurred to me, where on earth did this saying come from? We all have theories on "Break a leg" but I've never heard one for "chookas".

Any guesses?

By the way, has anyone heard anyone use "Break a leg" lately? All I've heard for the last few years is "chookas"...