After my umpteenth time hearing someone refer to a dangerously underweight person as "Skeletor," I feel compelled to step in and say, "Now I am Master of the Universe "Enough."
This is Skeletor:

As you can see, he's not a slender overlord. Ridiculously low body fat? Sure. Skinny face? Absolutely, but unless it's only in reference to one's head, using his name to deride an emaciated person is incorrect.
In the future, may I suggest the following to replace "Skeletor" when referring to bony adversaries:
Twiggys
Lara Flynns
McBeals
Pumpkin Kings
Harryhausens
You get the idea. Leave poor Skeletor alone.
He used to be a 98-pound weakling-monster before BALCO Labs got hold of him. After becoming buff and now brimming with roid-infused self-confidence, he applied for and got a job as a Pentagon media liaison.
Hey, Jason and the Argonauts! Yea! Harryhausen owns!
Can I please continue to refer to Ann Coulter as Skeletor? It's not so much because of the fact that she's underweight . . . more because she is the evil overlord of the dark side.
Oh man I love that show. I never missed an episode. I have the POWER!!!