This is the 9th most frequent Icelandic word.
því
"Því" means "therefore," "because of it," or "that" (in certain contexts).
Here, "því" is used as a conjunction meaning "because," showcasing causality.
Ég borða ekki fisk þvi honum líkar ekki.
I don't eat fish because I don't like it.
In this case, "því" is demonstrating consequence between the prior and subsequent statements.
Það var dimmt út því hann kveikti ljósin.
It was dark outside, so he turned on the lights.
Here, "því" is used in the adverbial expression "því miður" to express regret or misfortune.