tanko

The vehicle, and in Esperanto only the vehicle. The Reta Vortaro defines tanko as an armoured vehicle running on wide belts instead of wheels, and dates it to 1916. That is the whole entry. If you want the other English tank, the one that holds liquid, Esperanto sends you to cisterno, rezervujo or plain ujo. Writing la tanko de akvo for a water tank is an anglicism and a reader will picture something with a gun on it.

A cover name that stuck. The British built the first ones in secret in 1915, and the secrecy is why the word exists. The hulls were described on the paperwork as water carriers for Mesopotamia, workers were told they were tanks, and the name outlived the deception. Almost every language then borrowed the English word, Esperanto included, so the whole world now calls the machine after a lie told to a factory floor.

Where it sits in the vocabulary. Armilo is any weapon, kanono the artillery piece, and veturilo any vehicle at all; a tanko is specifically the armoured, tracked, gun-carrying one. Most of its appearances in the corpus that produced this frequency list are twentieth-century history, and the neighbouring mondmilito at rank 2245 tells you which century and roughly which decade.