The word tank was first applied in a military context to British “landships” in 1915 to keep their nature secret before they entered service.[3]
Origins
On 24 December 1915, a meeting took place at the Inter-Departmental Conference (including representatives of the Director of Naval Construction’s Committee, the Admiralty, the Ministry of Munitions, and the War Office). Its purpose was to discuss the progress of the plans for what were described as “Caterpillar Machine Gun Destroyers or Land Cruisers.” In his autobiography, Albert Gerald Stern (Secretary to the Landship Committee, later head of the Mechanical Warfare Supply Department) says that at that meeting:
Mr. (Thomas J.) Macnamara (M.P., and Parliamentary and Financial Secretary to the Admiralty) then suggested, for secrecy’s sake, to change the title of the Landship Committee. Mr. d’Eyncourt agreed that it was very desirable to retain secrecy by all means, and proposed to refer to the vessel as a “Water Carrier”. In Government offices, committees and departments are always known by their initials. For this reason I, as Secretary, considered the proposed title totally unsuitable.[a] In our search for a synonymous term, we changed the word “Water Carrier” to “Tank,” and became the “Tank Supply” or “T.S.” Committee. That is how these weapons came to be called Tanks.
This is, in fact, the etymology.
https://en.wikipedia.org/wiki/Tank