From 16 November 1942 onwards, Anne Frank had to share her room with the dentist Fritz Pfeffer, who had a number of habits that Anne couldn't stand. In the German biography Anne Frank (Reinbek bei Hamburg: Rowohlts Taschenbuch Verlag, 2002), Matthias Heyl writes:
In ihrer überarbeiteten Tagebuchversion revanchierte [Anne Frank] sich damit, dass sie ihm den weder im Niederländischen noch im Deutschen schmeichelhaften Namen «Dussel» verpasste.
In the reworked version of her diary, [Anne Frank] took revenge on him by giving him the nickname "Dussel", which is unflattering, both in German and in Dutch.
(More literally translated: "flattering neither in German nor in Dutch".)
But as far as I know, dussel is not a word in Dutch (my native language). So why would the name be unflattering in Dutch? I am looking for evidence in a reliable offline source; I have seen too much nonsense on the Web.