a "bad penny"

Where does the expression about a "bad penny" come from what does it mean, what is the real expression anyway

The expression I always knew invariably went along the lines of "he's turned up like a bad penny",
i.e. he's unwelcome, just as a "bad"(counterfeit) penny was unwelcome if given to you in change. Remember, the saying must be very old, certainly going back to times when a penny was worth a bit!