A plack (Scottish Gaelic: plang) was an ancient Scottish coin of the value of four Scots pennies or by 1707 one third of an English penny. It appears in the old song:
The word is probably derived from the ancient Flemish coin, a plaquette, current before the introduction into the Netherlands of the French money reckoned by francs and centimes.
It can be found in the works of Robert Burns too:
Nae howdie gets a social night,(Scotch Drink)Stretch a joint to catch a plack,Abuse a brother to his back.(To Gavin Hamilton)References
Plack (coin) Wikipedia(Text) CC BY-SA