Sin Skin is a slang term in many christian circles for the foreskin, or penis in general. The word developed from a satirical observation of the evangelical tendency to portray masturbation as something evil or taboo. The word is usually used in a jocular sense when discussing self gratification.
'John. I am totally going to touch my sin skin tonight'
'You better not Simon. Remember you will be possessed by the devil if you do.'
'hahahaha' from both Simon and John as they laugh at the subject matter, and satirical tone, of the conversation.