Yiddish for oh (oy) powers. The word gevalt comes, in turn, from the German gewalt meaning "power"

A general-purpose exclamation, Much like oy vey. Might be loosely translated as "Powers that be, help me!"

You don't hear it as much as oy vey, most likley because gevalt doesn't slide off English tongues as smoothly as vey. But Number Two does say it in Austin Powers, and I've come across it in a number of novels.