When you link someone to a website they have already seen. It refers to a fictional top-level domain (like .com, .net and .edu) that, if it existed, would house all the old shit everyone has seen but you.
Guy1: Have you see tubgirl?
Guy2: you mean www.tubgirl.old?
Example 2 - The lazy approach
Guy1: Hey, check out todays woot Guy2: .old