It doesn't help much that the only dictionary I have access to is an internet one - which means it isn't as complete as a big paper dictionary. It doesn't have "megin" as an adverb, but only as a noun. Long live forums like these where knowledge can be shared !
PS: Je vois que nous sommes principalement des francophones à nous intéresser à la langue islandaise.
Etrange phénomène !