Noun. a Watschn is An open-handed slap, also known as a smack or wallop. Origin unknown, but most likely onomatopoeia for the meaty sound of a square smack in the face. Generally used in plural (they seldom travel alone), Watschen are a Viennese slang institution to the point that it’s a generic term for physical violence, as in “Wüst a Watschn?” (“Wanna get slapped?”), superseding the technically more accurate Schläge (blows) or Prügel (a beating).
Most frequently mentioned in schoolyard or dive bar settings, Watschen figure prominently in a large array of threats and even have several synonyms, like Flak (after the infamous German anti-aircraft gun) or Tschinön, a corruption of the Italian word cinello (cymbal). One of the most common and colorful uses is in the expression “Du rüttelst am Watschenbaum” (lit. “you’re shaking the slapping tree”); if you ever hear someone ominously declare that, best stop antagonizing them – or you just may harvest the fruits of your labor.