congius

1. (Abbr. c. or cong.) Pharmacology. A gallon.
2. An ancient Roman measure for liquids, equal to about seven eighths of a U.S. gallon (3.3 liters).

[Middle English, a liquid measure, from Latin, from Greek konkhion, diminutive of konkhē, konkhos, shellful.]

The noun has one meaning:

Meaning #1: a British imperial capacity measure (liquid or dry) equal to 4 quarts or 4.545 liters
  Synonyms: gallon, Imperial gallon