The Collaborative International Dictionary of English v.0.48:

Custos \Cus"tos\ (k[u^]s"t[o^]s), n.; pl. Custodes
   (k[u^]s*t[=o]"d[=e]z). [L.]
   A keeper; a custodian; a superintendent. [Obs.]
   [1913 Webster]

   Custos rotulorum (r[o^]t`[-u]*l[=o]"r[u^]m) [LL., keeper of
      the rolls] (Eng. Law), the principal justice of the peace
      in a county, who is also keeper of the rolls and records
      of the sessions of the peace.
      [1913 Webster]

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

CUSTOS ROTULORUM, Eng. law. The principal justice of the peace of a county,
who is the keeper of the records of the county. 1 Bl. Com. 349.