Origin of the Military Tank

The word Tank comes from the First World War British code-name used for tanks when they were under development in 1915 and subsequently used when shipping them around, partly because under canvas they resembled large water containers, and partly because such a word was felt would seem reasonable to enemy code-breakers, given that desert warfare activities would require large water-containing tanks.

(various sources)

R Lee Ermey did a Mail Call episode on this in his first season.

and they were soppused to be "mobile water tanks".....