A person (usually a girl) trying to fuck someone so that they can reassure themselves that that look good/are still skinny enough for others to desire them.
"Did you hook up with John last night?"
"Yeah, but it was just a skinny fuck. It didn't mean anything and now I feel better knowing at least John still wants to fuck me."
The result of a drunk male attempting to insult another male on a Saturday night. The 'skin' part was due to the insultee weighing less than the insulter (a logical way to put someone down), the 'fuck' element used as it is always used in insults.
A Skinnypack is when a skinny male is so conceited and skinny that they think they actually have a six pack when really there is just so little skin the abdominal muscles are more visible.