The word tank was first applied to the British "landships" in 1915, before they entered service, to keep their nature secret. There are at least three possible explanations of the precise origin of the term:

1. One is it first arose in British factories making the hulls of the first battle tanks: workmen and possible spies were to be given the impression they were constructing mobile water tank for the British Army, hence keeping the production of a fighting vehicle secret.

2. Another is the term was first used in a secret report on the new motorised weapon presented to Winston Churchill, then First Lord of the Admiralty, by British Army Lt.-Col. Ernest Swinton. From this report, three possible terms emerged: cistern, motor-war car, and tank. Apparently tank was chosen due to its linguistic simplicity.

3. Perhaps the most compelling story comes from Churchill's authoritative biography. To disguise the device, drawings were marked "water carriers for Russia". When it was pointed out this might be shortened to "WCs for Russia", the drawings were changed to "water tanks for Russia". Eventually the weapon was just called a tank.

Source: Wikipedia