An ancient Greek liquid measure equivalent to about 40 litres (approx. 9 gallons).
In Middle English said to be the equivalent to 2½ quarts (2.8 litres): see Middle Eng. Dict. at that entry.
Late Middle English. From classical Latin metrēta and its etymon ancient Greek μετρητής from μετρεῖν to measure (from μέτρον) + -της, suffix forming agent nouns.