I made this word one day. It just came to me like magic.
When someone is so skinny and haggered they can barely function, in a healthy manner. Bones protrude and the skeleton is easily visible. This condition often is used with super models and heroin sheik videos, or due to drug usage. (i.e. Fiona Apple)
"daaayum, she's hella thin!"
"Nah man, She's skelethin!"