Lewis & Short

rătĭōcĭnātor, ōris, m. [ratiocinor], a reckoner, computant, accountant, i. q. calculator.

  1. I. Lit., Cic. Att. 1, 12, 2; Col. 3, 3, 7; Dig. 14, 4, 5, § 16.
  2. II. Trop.: ut boni ratiocinatores esse possimus et addendo deducendoque videre, quae reliqui summa fiat, Cic. Off. 1, 18, 59; Aug. Civ. Dei, 21, 5.