As weird as this saying is, it’s quite a common one used by Swedes. Obviously describing a troublesome, embarrassing situation, it would equate in English to something like ‘to be caught with your hand in the cookie jar’ or ‘caught with your trousers down’. In other words, to find yourself in a difficult situation of your own making.
But where does the expression come from?
One common theory is that it is from a 1959 book called Bitter Pills. The Swedish translator translated the English ‘who will get hurt’ to ‘who will sit with their beard in the letterbox’. Rather an odd translation one might think but actually it was rather a clever one.
The translator based his expression on a nautical saying at that time – ‘to fasten with your beard in the block’. Apparently a block is a wheel that mooring lines run through on a boat. I guess it would be very unfortunate for a sailor to get their beard caught in it while wrestling with a wild boat. The theory is that the translator wanted to modernize the expression, and use a bit of humour. So block became letter box. The expression can also be ‘to fasten with your beard in the letter box’.