the word 'tank' entered english in the early 17th century from portuguese 'tanque' or spanish 'estanque', both meaning 'pond' or 'reservoir', derived from the latin 'stagnum' (pool, swamp). its initial english meaning was an artificial pond or a large container for liquid. the military sense originated as a code name during world war i to maintain secrecy during the development of the new armored vehicle; the components were misleadingly described as "water carriers" or "tanks" for the mesopotamian campaign. this deliberate misdirection led to the permanent adoption of the name for the weapon, while the verb sense, meaning to perform poorly or collapse, is a 20th-century development, likely from the imagery of a tank emptying or an engine "tanking" (failing).

the word is a simplex loanword. its modern senses developed through semantic extension from its original borrowed meaning.