[of Yorkshire]. Same as trithing in Lincolnshire; the jurisdiction of a third part of a county, under the government of a reeve (sheriff). The word ding or thing is Scandinavian, and means a legislative assembly; hence the great national diet of Norway is still called a stor-thing (great legislative assembly), and its two chambers are the lag-thing (law assembly) and the odels-thing (freeholders' assembly). Kent was divided into laths, Sussex into rapes, Lincoln into parts. The person who presided over a trithing was called the trithing-man; he who presided in the lath was called a lath-grieve.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894