The expression originated from the Hebrew phrase "ben elef". This phrase is used to say "Son of a bitch/whore" without saying "bitch/whore". The idea is that if one's mom is a whore then there are thousand men that may be his father.
A: "You are fired!"
B: "Oh yeah, and You are a son of a thousand"
A: "You know my mom? are you one of her clients too?"