In New England, the adjective has transformed itself into an adverb. This is common in 19th century English. Take for example the use of "awful" - it has been transformed into the adverb, "awfully". This has a similar meaning to the adverbial use of "wicked," both meaning "really" or "very". You may hear someone exclaim that they're "awfully tired," anyone would understand "awfully's" use as an adverb intensifier to signify the strength of their fatigue.. A similar thing has happened with "terrible," "real" and "pretty". In what could be the same way, "wicked" has transformed itself into an intensifier in New England.

Hence "Something Wicked (Very) Cool This Way Comes.