trithing
a riding (one of three ancient divisions of a county in England)
historical
Where a county is divided into three of these intermediate jurisdictions, they are called trithings, which were antiently governed by a trithing-reeve. These trithings still subsist in the large county of York, where by an easy corruption they are denominated ridings; the north, the east, and the west-riding.
Where it comes from
From Middle English trithing, tridinge, from Old English *þriðing, from Old Norse þriðjungr (“third part”).
Adapted from the Wiktionary entry trithing (CC BY-SA 4.0).