Most fascinating word. Removing the ending 's' yields more hits though still < 700 on teh interwebz. Most are medical references to the muscular system but an odd handfull do indeed use it as a synonym for thin/skinny/anorexic, including a couple on DeviantArt. Odd to find only a tiny number of descriptive - as opposed to technical usages . It doesn't seem to be archaic or non-English,  just extremely uncommon with no formal definition listed anywhere that I see. My favorite reference:

"I don't think such a squeletic person can survive working has a prostitute ! For sure she doesn't swallow !"

At any rate, feed the hungry and improve your vocabulary at:

http://www.freerice.com

where I have yet to encounter squeletic.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken