Lewis & Short

arbī̆trārĭus, a, um, adj. [arbiter].

  1. I. Of arbitration, arbitrating, done by way of arbitration: formula, Gai Inst. 4, 163: actio, Dig. 13, 4, 2; cf. Zimmern, Rechtsgesch. 3 B, §§ 67 and 68.
    Hence,
  2. II. Transf.
    1. A. In Plaut. (with ref. to the distinction in law lang. between certus and arbitrarius: judicium est pecuniae certae, arbitrium incertae, Cic. Rosc. Com. 4; cf. Zimmern, Rechtsgesch. 3 B, § 57) = incertus, uncertain, not sure: hoc certum est, non arbitrarium, Plaut. Am. 1, 1, 216.
      Adv.: arbī̆trārĭō: nunc pol ego perii certo, non arbitrario, there’s no mistake about it, Plaut. Poen. 3, 5, 42 (the only adv. of this word in use).
    2. B. Depending on the will, arbitrary (cf. precarius): motus in arteriā naturalis, non arbitrarius, Gell. 18, 10 fin.