A "thin boy" is an item of clothing that a person has used and loved so much that it is worn down to a thin layer of material. For some reason, the production of these items usually seems to date back to before 1990.
Danny: "wow, that shirt looks like it is from 1988. I'm surprised it's still intact."
Kenny: "Yeah, it's my thin boy. I'll be sad when I have to replace it."
November 12, 2010