The Collaborative International Dictionary of English v.0.48:

Congius \Con"gi*us\, n. [L.]
   1. (Roman Antiq.) A liquid measure containing about three
      quarts.
      [1913 Webster]

   2. (Med.) A gallon, or four quarts. [Often abbreviated to
      cong.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

congius
    n 1: a British imperial capacity measure (liquid or dry) equal
         to 4 quarts or 4.545 liters [syn: gallon, Imperial
         gallon, congius]