Worthless.

Plugs are the holes made in coins, which is then filled with a cheaper metal. Coins so tampered with are no longer legal tender and are thus worthless if spotted. The phrase is, of course, American. Before 'plugged nickels' there were 'plugged quarters' and 'plugged dimes'. The various versions of the phrase appear in the 1880s. The nickel, being a lower denomination coin, lends itself better than quarters and dimes to a phrase expressing worthlessness. Oddly though, the lowest denomination coin is the cent and the phrase 'not worth a plugged cent' doesn't appear until later. The earliest I've found for that is 1908.

[For those not familiar with US coinage; a quarter is 25 cents, a dime is 10 cents and a nickel is 5 cents.]

The earliest of any version of the phrase that I can find is from The Daily Nebraska State Journal, 14th September 1883. This indicates the illegitimate nature of plugged coins:

"No," said a Philadelphia conductor, "I never attempt to pass a plugged quarter on a man unless he's got his Sunday girl with him. Then he's afraid she'll think him mean if he get's mad.