The "tank" in think tank - is it a fish tank or a battle tank? [duplicate]

This morning I read about a think tank somewhere having this or that opinion on some policy.

I suddenly had this image in my head of brains free-floating in some kind of big fish tank, exchanging thoughts and ideas to come up with something brilliant.

Then I remembered "tank" having other possible meanings in English. Like the battle tank, designed to overcome evil by brute force, armored to sustain heavy enemy fire.

And I wondered - is this what a think-tank is? Some massive brainy killing-machine, immune to outside agression, designed to come up with the one great idea that will win the war?

If anyone could point out what kind of tank was initially meant at creation of the first think-tanks I'd be very thankful.

Solution 1:

The original meaning of "think tank" according to the OED is:

The brain as the organ of thought; (hence also, by extension) the head. Now rare.

OED's earliest example is from 1889, although this answer finds examples from 1887. This means it's certainly not related to the armored vehicle "tank", which only dates back to 1916. Instead, the right definition of "tank" is:

a large receptacle or storage chamber, esp. for liquid or gas.
_{Oxford Dictionaries}

OED makes a note that "think tank" is pretty similar to the expression "thought box" (first attested in 1801) or the expression "think box" (first attested in 1910).

The Blackwell Encyclopedia of Sociology explains how "think tank" shifted in meaning:

A shift in the term's referent, from brain to research organization, began in the late 1950s. One organization, the Stanford University-based Center for Advanced Study in the Behavioral Sciences (CASBS), appears to have supplied the bridge for this shift. CASBS acquired the nickname “the Think Tank” (usually capitalized) around 1957 for its high concentration of “brainpower” and technical proficiency. During the 1960s, the term came to be associated with a set of post-World War II military planning groups.

Although the modern definition is slightly different, it's pretty clear that it's related to the sense referring to research groups.