The Random House Dictionary defines the expression think tank as:
- a research institute or organization employed to solve complex problems or predict or plan future developments, as in military, political, or social areas.
According to etymonline the above connotation is an extention of an earliar sense which referred to "the brain":
- also think-tank, 1959 as "research institute" (first reference is to Center for Behavioral Sciences, Palo Alto, Calif.); it had been colloquial for "the brain" since 1905.
Ngram: shows usage of "think tank" only from the late '50s.
Is there evidence of earlier usages of "think tank" meaning "the brain"? Despite its colloquial usage I assume the expression was quite widespread before it was adopted by the Stanford University.