Where does the phrase "always comes back like a bad penny" come from? And what does it mean?

I think that the phrase to which are referring might be the following one.

A bad penny always turns up

The proverb, also used allusively in simile and metaphor (see quot. 1766), refers to the predictable, and usually unwelcome, return of a disreputable or prodigal person after some absence.

Like a bad penny it returnd, to me again.
[1766 A. Adams in L. H. Butterfield et al. Adams Family Correspondence (1963) I. 55]

Bring back Darsie? little doubt of that—the bad shilling is sure enough to come back again.
[1824 Scott Redgauntlet II. ii.]

Just like as not he'll be coming back one of these days, when he's least wanted. A bad penny is sure to return.
[1884 R. H. Thorpe Fenton Family iii.]

Who's dead, when and what did he die of? Turn up like a bad penny.
[1922 Joyce Ulysses 149]

'I miss Bart.' 'Oh, a bad penny always turns up again.'[1941 A. Updegraff Hills look Down vi.]

'Stop worrying. The bad pennies always turn up.' 'Oh, Adrian, I don't think she's a bad penny, not really.'[1979 G. Mitchell Mudflats of Dead iii.]
Related to: wrong-doers

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Learn English Idioms - Idioms and Sayings About Money (http://www.learnenglish.de/vocabulary/moneyidioms.htm)
Originally a bad penny always turns up

Our mistakes return to haunt us; also, nasty people have a way of reappearing.

Started around 18th century
One of last famous times quoted Indiana Jones (Harrison Ford) in the Last Crusade "I'm like a bad penny,"

So what is a bad penny?