What is "quass"

Quass \Quass\, n. [Russ. kvas'.] A thin, sour beer, made by pouring warm water on rye or barley meal and letting it ferment, -- much used by the Russians. Called also kvass. [written also quas.]

n. (alternative spelling of kvass English) vb. (label en obsolete) to drink deeply; to quaff.