tankard n.

late 14c., "large tub-like vessel," corresponding to Middle Dutch tanckaert, meaning the same thing, but both of unknown origin. A guess hazarded in OED is that it is a transposition of *kantard, from Latin cantharus. Klein suggests French tant quart "as much as a quarter." "The notion that the word is from tank 1 + -ard is wholly untenable" [Century Dictionary]. Meaning "drinking vessel" is first recorded late 15c.