A tankard is a form of drinkware consisting of a large drinking cup that usually features a single handle, usually of silver or pewter construction. Tankards featuring glass bottoms are also fairly common.
The legend is that the glass bottomed tankard was developed as a way of refusing the King's shilling. i.e. conscription into the British army or navy. The drinker could see the coin in the bottom of the glass and refuse the drink, thereby avoiding conscription.