An adjective. It means to look halloween-like.

a person in highschool or older who still goes trick-or-treating instead of going to a party.

A person who loves Halloween above all holidays. Halloween is their Christmas, Easter, Valentines and St.Patrick's day all-in-one. Of course this person probably dresses mostly in black, could be kinda scary themselves until you get to know them, makes cool props, decorates their yard and home like a haunted house and has no problem coming up with the most creative and original costumes ever to show up at a party.
(Definitely an interesting person to know in my humble opinion!!)

Someone who dresses up as someone/something based on an arcane cultural reference that no one would get, for the sole purpose of showing off how much of an intellectual they are

Describing something with an overwhelming Halloween aesthetic.

or/

Something that reminds you of Halloween.

The genitalia of a male skeleton (females have a hallowenus). Although many will argue that this is impossible for a skeleton to have a hallowenie as a weenie “does not have bones”, this is obviously a lie created by the government.

Holiday celebrated with “dick or treat”, and many latex ghost and birthday suit costumes.