Adjective. 1. Dumb, moronic, idiotic, stupid; 2. Crazy, unhinged, nuts. Orig. from the German word Depp (half-wit, moron, simpleton), which has been in use since at least the 15th century, which is derived from the older täppisch (clumsy, ungraceful), which in turn can be traced to the medieval Middle High German tāpe (paw), implying that someone is moving as if they had paws instead of hands – hence clumsy.
In everyday Viennese usage, deppert is simply one of the many ways to dismiss someone as crazy, stupid or both – the staple of insults worldwide.
It follows that there are many derivates of deppert – a Depperte(r) would be a stupid (or crazy) person; schneedeppert (lit. snow crazy) is used to describe excessive excitement about the fresh white powder; and Bist du deppert! (are you stupid!) is a popular exclamation expressing surprise mixed with admiration and/or contempt, roughly analogous to the English “Are you kidding me?”
One of the most famous uses of the word in recent years came from long-time mayor (and walking Viennese institution) Michael Häupl, who commented on the city’s good turnout for his Social Democratic party (and poor results for the rival far-right FPÖ) during the 2017 National Assembly election by proudly declaring, “Mei Wien is ned deppert.” (My Vienna ain’t stupid)