Other forms: tanks, tanking
A tank is a large container for liquid or gas — a fuel tank, a water tank, a fish tank. It is a container built to hold something under pressure or in quantity, which is what separates it from a box.
The armoured vehicle is the same word and the reason is a good story. It also works as a verb meaning to fail badly: the film tanked, sales tanked, and that use is informal and American in origin but now general. Think tank is a fixed compound for a research institute, and it has nothing to do with either container or vehicle — the tank there is the head.
1nountankA large container for liquid or gas.
“The water tank is in the loft.”
2nounarmouredAn armoured fighting vehicle running on tracks.
“A column of tanks came through the town at dawn.”
3nounglassA glass container for keeping fish.
“There is a tank of tropical fish in the waiting room.”
4verbfailTo fail badly.
“The share price tanked after the announcement.”
Too few real examples of this word to estimate which sense is commonest.
tank is a large container.
Tank came into English from India, from a word for a cistern or reservoir. The armoured vehicle got the name as wartime cover: the first ones were shipped in crates labelled as water tanks for Mesopotamia, and the disguise stuck to the machine permanently.
Portuguese
tanque
English, today
tank
Built from
Borrowed from Portuguese tanque. From Portuguese tanque (“tank, liquid container”), from an inc language, likely gu ટાંકી (“cistern”) or mr टांकी. A false friend can be found in gmq: dunk (“canister, keg”), 17th c. tönk, which appears derived from both tunneke, tonneke, and dunka (“to dunk, to dip under water, to moisten”).
Etymology from Wiktionary, CC BY-SA