I've heard this phrase in the "Grand Budapest hotel" movie and understand that it means 'to be very curious'. But what does "Flitzebogen" means? Is it a common expression? In which cases is appropriate to use it?
This is a pun.
A Flitzebogen is a toy bow. To draw or bend a bow is in German einen Bogen spannen. So the bow is drawn/bent is in German der Bogen ist gespannt.
One valid translation of I am curious about something is Ich bin auf etwas gespannt.
So in the sentence »Ich bin gespannt wie ein Flitzebogen« the word »gespannt« has the two meanings curious and bent at the same time.
I am curious/bent like a toy bow.
curious and bent: the German words "spannen, gespannt, Spannung" correspond to the English words (to) tension, (to) tense, which can also have both meanings.
"toy" bow does not correspond to ""Flitze"bogen", even if children's bows are belittling called "Flitzebogen". This comes from the colloquial verb "flitzen", which means to be on the road very fast, whether on foot or by car or as a "Flitzer" (streaker). I found the following translations: whip, whiz, streak, hare, scoot, dart, dash. It is, as already mentioned above, a very tense bow, a bow that can shoot fast or soon.
The more useful explanation in this context would be:
The bow is strained. He is about to leave. The tension curve is briefly tearing. It describes the situation just before the release, at which the tension is the largest.
Since curiosity works, you are very curious about what comes and the tension is bigger the closer you are to the information.
If a friend tells you a story and this tends to the climax, or someone tells you a joke and is close to the punchline.