What is a bad penny in relationship to the common phrase "he/she keeps turning up like a bad penny"?

if "he/she turns up like a bad penny" they come in, and talk about nothing about her/him self

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I think this is probably a reference to tossing a coin to decide something. A penny is a type of coin, and the old British penny, in use up to about 1970, was a big coin, about 30mm across, so it was very good for tossing to decide a 50/50 outcome. A bad penny would be one that would keep turning up the same side.

When a counterfeit or bad penny was in circulation, anyone finding it in their change would get rid of it at the earliest opportunity. Thus it would circulate much more rapidly than a good penny, and would keep turning up, unwanted.

"BAD PENNY -- The phrase usually is heard in this country (U.S.) as 'A bad penny always turns up,' meaning that a no-good person can be counted upon to come back again and again. The expression was originally English and the unit of currency referred to was the shilling. Sir Walter Scott, in one of his early nineteenth-century novels, whereto: 'Bring back Darsie? Little doubt of that. The bad shilling is sure enough to come back again.'" From "Morris Dictionary of Word and Phrase Origins" by William and Mary Morris (HarperCollins, New York, 1977, 1988).

I've never heard the expression 'bad shilling' in UK use. That's probably a peculiarity of Scott's. (SOme authors do rewrite well-known expressions - Somerset Maugham is notorious for it.) The bad penny probably dates from the days of the silver penny (copper started to come in in 1797 - made by Boulton and Watt, even, and weighing a full ounce) and at times the quality of the silver in them was not up to what people thought it should be. As Dave says, if you got one of these duff pennies, you would spend it as quickly as you could. And by a certain law, it was bound to come back to you in change again, especially in smaller towns, where the supply of money was more restricted.

Not answering the question there... As zbeck says, the person turns up when not wanted - and frequently at that.

Thanks guys that all makes sense