The Collaborative International Dictionary of English v.0.48:

Cong \Cong\, n. (Med.)
   An abbreviation of Congius.
   [1913 Webster]

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]