The Collaborative International Dictionary of English v.0.48:

Tubman \Tub"man\, n.; pl. Tubmen. (Eng. Law)
   One of the two most experienced barristers in the Court of
   Exchequer. Cf. Postman, 2.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Tubman
    n 1: United States abolitionist born a slave on a plantation in
         Maryland and became a famous conductor on the Underground
         Railroad leading other slaves to freedom in the North
         (1820-1913) [syn: Tubman, Harriet Tubman]

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

TUB-MAN, Eng. law. A barrister who has a pre-audience in the Exchequer, and
also one who has a particular place in court, is so called.