noun Scots law (formerly)

an obligation imposed upon tenants of certain lands requiring them to have their grain ground at a specified mill
the fee paid for grinding the grain

Word Origin for thirlage

C16: variant of earlier thrillage, from thrill, Scottish variant of thrall
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Examples from the Web for thirlage

Historical Examples of thirlage