What's the "een" in Halloween?
Old English into, originally in to. The word is a late Old English development to replace the fading dative case inflections that formerly distinguished, for instance, "in the house" from "into the house." To be into something, "be intensely involved in," first recorded 1969 in American English.
Currently interested or involved in; now practicing or absorbed in: a former Ivy Leaguer named Crimpcut who is into Buddha/Cool it, woman, I'm inta my thang/if you're into Chinese cuisine (1960s+)