I was curious to where the word "tank" comes from too, so I looked I looked it up:

The word "tank" originally came from India, brought by the Portuguese. It referred to a pool or lake for irrigation or drinking water. The German word "panzer" means "armor" and is related to the French word "pancier" for "breastplate". The term "tank" was first applied to British "landships" in 1915 to keep their nature secret before they entered service.