A tank is a large container for holding liquids or gases, such as water or fuel. The word also refers to a heavily armored military vehicle that moves on tracks.
The container meaning comes from Portuguese 'tanque' meaning 'reservoir', likely from a word in India such as Gujarati 'tānkhi' for 'cistern'. The military vehicle was nicknamed 'tank' in World War I as a code word, pretending they were just water containers to hide their real purpose.
The secret code name for early armored vehicles—'tanks'—stuck so well that we forget it was originally a trick. That’s why one short word now covers both a peaceful water container and one of the most destructive machines humans have built.
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