Literally: “roof damage” / meaning: a mental disorder
The creativity of a language is largely defined by the quality of its insults. And of these, the German language has a great selection for every occasion and intensity. Especially for everyday swearing, there are some real gems on offer. For instance, if someone acts a bit crazy, he either doesn’t have all his cups in the cupboard (nicht alle Tassen im Schrank) or he is muddy in the pear (Matsch in der Birne). It might also be that he is not the brightest candle on the cake (nicht die hellste Kerze auf der Torte). Or, perhaps most commonly, he simply has a Dachschaden.
The Dach (“roof”) is clearly a synonym for head, as is also the case in the phrase jemanden eins aufs Dach geben (“to give someone something on the roof”), meaning to spank someone. There is also the lovely German phrase jemandem aufs Dach steigen (“to step on someone’s roof”), meaning to complain to someone in the harshest possible way. And in complaining, I can tell you, Germans are just as creative as with their insults.