According to the Dictionary of the Scottish Language, a plack was "a small Scottish coin, originally of billon but later of copper, issued by James III. c.1470, and later monarchs till the Union of the Crowns in 1603. It was valued at four pennies Scots or one third of an English penny."

See also the line "But ye must pay them plack and farthin' before on board ye go" in the song Caledonia (Roud 5543).