Popularized by Jack Skellington from Tim Burton's The Nightmare Before Christmas, but also used in place of skeleton as seen in Scarygoround, a webcomic by John Allison.
If someone is using a skeleton and/or skeletal avatar and/or signature on a forum, or can medically be described as a skeleton, they refer to themselves as Skellingtons in the hope of a small ego boost. Also used by n00bs when trying to describe a Skeleton.
While balls deep in a vagina from behind whilst wearing a skeleton costume and a pink sock starts sneaking its way out, quietly peel off an arm bone and jam it in her anus while you continue merrily on your way to ejaculation.
Last Halloween, I had to Jack Skellington this nasty hoe before her pink sock exploded on me.