Höstrusk is about as Swedish as a word can get, and doesn't just describe an undesired autumnal weather condition, it also encompasses the mood and feeling that goes with it.
Höstrusk best translates as 'nasty damp autumn weather' (literally, höst means autumn, and rusk means nasty or horrible) and is heavily used among Swedes in October and November to describe the rainy, cold, and grey period just after the trees lose their leaves but it's still too early for snow and Christmas lights.
The word has quite a negative ring to it and its application pretty much coincides with when the Swedes start spending much more time indoors (a period which generally lasts until the spring and has earned them the reputation of 'isolating' themselves during the winter months).
Swedish writer, poet and painter August Strindberg (1849-1912) definitely doesn't seem to have been a big fan of the Swedish höstrusk weather. In 1885, he wrote that “all that was left for me to do was to go on a journey in the midst of höstrusket” (För mig återstod således blott att midt i höstrusket företaga en resa).
Jag ger mig aldrig ut i det där höstrusket.
I'll never head out into that nasty autumn weather.
Njut av vädret innan höstrusket kommer.
Enjoy the weather before the nasty autumn weather sets in.
Det är skönt att vara inne när det är regn och rusk ute.
It's nice to be inside when it's rain and that horrible weather outside.