From jätte- + kul. jättekul (not comparable) (informal) very fun jättekul in Svenska Akademiens ordböcker...
Sometimes kulig is used colloquially when inflected forms are needed. jättekul kulig skitkul kula rolig skoj kul in Svensk ordbok (SO) kul in Svenska...
to blame – Jag vill inte följa med. – Skyll dig själv. Det kommer bli jättekul. – I don't want to go. – Your loss / Suit yourself ("Blame yourself" (for...