A third person or party; esp. one called in as an intermediary, mediator, or arbiter.
1818. Scott, Hrt. Midl., xxiv. There was risk of Andro Ferrara coming in thirdsman. Ibid., xlviii. if I come in thirdsman among you at the kirk-sessions, you will be all in a tamn’d pad posture indeed.
1887. Saintsbury, Hist. Elizab. Lit., x. (1890), 386. Herrick and Carew … with Crashaw as a great thirdsman, called themselves ‘sons’ of Ben Jonson.
