Läskig means 'scary' or 'spooky', and can be used to describe a film, impressive Halloween costume, or an experience. So if any children come to your door this evening dressed as zombies or witches and asking bus eller godis? ('trick or treat?' or literally 'trick or candy?'), you might tell them du ser jätteläskig ut! (you look really scary).
It used to be spelled ledskig and, before that, ledskug. In case you were wondering, there's no relation to the Swedish word for fizzy drinks or soda, läsk. This is a shortening of läskedryck (soft drink), and comes from the verb läska which means 'to quench one's thirst' and itself comes from the Low German word leschen (to extinguish).
But that doesn't stop soda companies capitalizing on the opportunity for wordplay, so don't be surprised if you see a lot of spookily-themed drinks adverts around this time of year.
Because of this existing meaning, you can't use the verb läska to mean 'to scare/frighten'. Instead you have to use skrämma (which has its own adjective form, skrämmande, also meaning 'scary').
Barnen såg en läskig film
The children watched a scary film
Det ser läskigare ut än vad det är
It looks scarier than it really is