@be @jens contextually I read it as an onomatopoeia, as you intended. However, now that Jens mentions it, the German meaning of the word is used here too, but the usage is quite a bit softer than "stupid". More closer to the way weebs use the word "baka". Eg sometimes someone will say "I'm a doof" upon realizing they made a lapse of judgement or forgot an important piece of information and jumped to the wrong conclusion, or less commonly call a friend a doof when they're acting confused.