"It hadn't penetrated my think-tank that this was your hacienda when I came mavericking in."

Wrote William MacLeod Raine in the Western novella "Bucky O'Connor" (1910). 

The original meaning of "think tank" was brain. I learned that just now from the OED. 

In 1964, the St. Louis (Missouri) Post-Dispatch quoted Harry Truman saying that he wants to live to be 90 "if the old think-tank is working."  I guess the skull is the container — the tank — and the brain the contents. I see that people also said "think box." 

Does anyone still use the phrase "think tank" like that? It would be confusing, now that "think tank" has come to mean "A research institute or other organization providing advice and ideas on national or commercial problems" (OED). 

The oldest published appearance of that usage is:

1958 Econ. Jrnl. 68 362 The Center for Advanced Study in the Behavioral Sciences (known in some quarters as the Think Tank) at Palo Alto.

The parenthetical was needed. Shortly thereafter, quotation marks eased the term into our vocabulary:

1962 N.Y. Times 3 Nov. in D. L. Larson Cuban Crisis 230 Robert Kennedy had stepped out of a ‘Think Tank’ meeting that morning to return a call from the President.

1967 Mrs. L. B. Johnson White House Diary 8 Oct. (1970) 577 Mt. Hope Farm..will be the site for the environmental planning center—a sort of a ‘think tank’ for city-planning experts. 

By the time Nixon took over, the scare quotes were gone... or were moved over to a coinage of Nixon's, "brainwork":

1969 Hutchinson (Kansas) News 22 Jan. 1/4 He [sc. President Nixon] said he is going to..take over a smaller room across the street as a kind of think tank for what he called ‘brainwork’.

Brainwork — for thinking — that never caught on. Or did it already exist and its association with Nixon killed it? 

I see an OED entry for "brainwork," and it appeared in print in 1606: "Oh Philocalia, in heauy sadnes & vnwanton phrase there lies all the braine worke, by what meanes I coulde fall into a miserable blanke verse presently." 

And I like this (from 1703): "I am fully convinc'd that Brain-work infeebles the Body extreamly."