What is Thirlage?

Legal Definition
Thirlage was a feudal servitude (or astriction) under Scots law restricting manorial tenants in the milling of their grain for personal or other uses. Vassals in a feudal barony were thirled to their local mill owned by the feudal superior. People so thirled were called suckeners and were obliged to pay customary dues for use of the mill and help maintain it.
-- Wikipedia
Legal Definition
Scotch law. The name of servitude by which lands are astricted or thirled to a particular mill, and the possessors bound to grind their grain there, for the payment of certain multures and sequels as the agreed price of grinding. Ersk. Prin. B. 2, t. 9, n. 18.
-- Bouviers Law Dictionary
Legal Definition
In Scotch law. A servitude by which lands are astricted or "thirled" to a particular mill, to which the possessors must carry the grain of the growth of the astricted lands to be ground, for the payment of such duties as are either expressed or implied in the constitution of the right Ersk. Inst 2, 9, 18.
-- Black's Law Dictionary
Legal Definition
(Scotch) The service of payment for the grinding of corn.
-- Ballentine's Law Dictionary