What's a plugged nickel?

[CookingWithGas]

In the column about hanging bags of water to repel houseflies (http://www.straightdope.com/mailbag/mwaterbags.html), it say that some insects' eyesight is not worth a plugged nickel. I've heard it many times. . .but what is it?

[lno]

http://www.wordorigins.org/wordorp.htm#Plugged
This is US slang dating to 1888. Back when coins actually contained valuable metal, it was a counterfeiting practice to remove the silver or other valuable metal from the center of the coin and replace it with a plug of lead or other base metal. Hence, plugged nickel.

[hegel]

to "plug" is to remove a coin's center. 'round these parts, it's common for a coin to be plugged by, say, a .22...in earlier times, the center of a coin was often made of gold or some other precious metal. dishonest folks would take out the center and replace it with something less valuable.

[Johnny L.A.]

[pirate voice]I says we plug 'im, an' see if 'e's ripe![/pirate voice]