What is the X in X-mas?
c.1300, "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. Meaning "drinking vessel" is first recorded late 15c.
drinking vessel for ale or beer, widely used in northern Europe (especially Scandinavia, Germany, and the British Isles) and in colonial America from the second half of the 16th century until the end of the 18th century. The body is usually cylindrical, and it has a hinged lid (with or without finial), generally a thumbpiece, and a handle that is often in a scroll shape. On the handles of many 17th- and 18th-century tankards are whistles that were used for summoning waiters. Though they were sometimes made of horn, carved ivory, pottery, and porcelain (all with metal mounts), tankards most often were made from precious metals, especially silver, and pewter. In the late 17th century certain large tankards appeared, commissioned by corporate bodies or guilds for presentation or ceremonial use. Reproductions of traditional tankard styles date from the 19th century and continue to be produced in the 20th century.