The etymology here is actually really interesting. The term "tank" was an Americanism for a swimming pool once upon a time, and the etymology of that is pretty well-known: it was imported from a Portuguese word meaning "pond" which ultimately came from India [1]. The same source quotes a 1960s usage of the term for "failure" in the sport of Tennis.
The term actually seems to come from a 1920s boxing euphemism [2]: when a boxer is not actually knocked out but voluntarily lays down on the ground, it was called "a dive" for obvious reasons; euphemistically some people called this "going into the tank," since you'd dive into a pool.
How did this start referring to the vehicle? Again, back to [1], there was once a memo "recommending the proposed "caterpillar machine-gun destroyer" machines be entrusted to an organization "which, for secrecy, shall be called the 'Tank Supply Committee,' ..." and the rest is history.
The term actually seems to come from a 1920s boxing euphemism [2]: when a boxer is not actually knocked out but voluntarily lays down on the ground, it was called "a dive" for obvious reasons; euphemistically some people called this "going into the tank," since you'd dive into a pool.
How did this start referring to the vehicle? Again, back to [1], there was once a memo "recommending the proposed "caterpillar machine-gun destroyer" machines be entrusted to an organization "which, for secrecy, shall be called the 'Tank Supply Committee,' ..." and the rest is history.
[1] http://www.etymonline.com/index.php?term=tank
[2] http://www.slate.com/articles/news_and_politics/explainer/20...