means A tank is either a large container for holding liquid or gas, or an armored, tracked combat vehicle bristling with weapons.

from The container sense comes through Portuguese 'tanque' (pond, reservoir), likely borrowed from an Indian language — Gujarati 'tankh' or a related word for a man-made water cistern. The war machine got its name as a deliberate cover story: when Britain built the first ones in WWI, the project was disguised as the manufacture of mobile 'water tanks,' and the codename simply stuck to the steel beast.