Answer: Landships

Given that, outside of military usage, the word “tank” designates a holding container of some sort (such as the gas tank of a car or tanker truck that carries liquids in a giant holding tank), it seems odd that the word also means, in military usage, a giant armored land vehicle.

What we now call tanks were, in their original incarnation as designed by the British during World War I, known as “landships”. Internally, within documents exchanged between designers, the military, and political leaders, the armored vehicles were referred to as landships. In order to keep the nature of the operation secret, however, workers in factories and other laymen that came in contact with the vehicle construction programs were told they were building transportation vehicles intended to haul water and other liquids: mobile tanks, if you will. The code name proved far catchier than the original name; it’s stuck ever since.