The Collaborative International Dictionary of English v.0.48:

Thirlage \Thirl"age\, n. [Cf. Thrall.] (Scots Law)
   The right which the owner of a mill possesses, by contract or
   law, to compel the tenants of a certain district, or of his
   sucken, to bring all their grain to his mill for grinding.
   --Erskine.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

THIRLAGE, 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.