Definition of "twiscar"

As for her brother, they being now arrived where the rude and antique instruments of Zetland agriculture lay scattered in the usual confusion of a Scottish barn-yard, his thoughts were at once engrossed in the deficiencies of the one-stilted plough—of the twiscar, with which they dig peats—of the sledges, on which they transport commodities—of all and every thing, in short, in which the usages of the islands differed from that of the main land of Scotland.

1822, [Walter Scott], chapter XII, in The Pirate. […], volume I, Edinburgh: […] [James Ballantyne and Co.] for Archibald Constable and Co.; London: Hurst, Robinson, and Co., pages 272–273