In other words, it means “fuck off” or “up yours.” However, if I grew up in Brazil or Germany, and my teacher gave me the A-OK sign, it would.
Watch yourself! as more of an obscene gesture, similar to saying "fuck you." When in doubt, palm-outward peace signs seem to be suitable wherever you are in the It's definitely not "OK" in Brazil, Germany, and Turkey.
You probably wouldn't learn that in most of your traditional German courses. you a rather disparaging insult, roughly equivalent to “fuck you!”.