Regarding the origins of the phrase/word 'Think Tank'; Here are some references from the Oxford english dictionay.
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Think Tank

1. colloq. The brain. U.S.

1905 A. L. STILLMAN in A. H. Shearer Little Bk. Rutgers Tales 51 There's too much scrapping in the Joint... Your Think-tanks are getting to be Air-tight Compartments. 1910 [see MAVERICK v. b]. 1964 St. Louis Post-Dispatch 8 May 2A/3 Truman..said he hoped to live to be 90 but only ‘if the old think-tank is working’.

    2. A research institute or other organization providing advice and ideas on national or commercial problems; an interdisciplinary group of specialist consultants. Also in extended (usu. facetious) use.

1959 Times Lit. Suppl. 6 Nov. p. xix/2 Even the Institute of Advanced Studies at Princeton does not quite meet the bill, nor does the ‘think tank’, the Center for Behavioral Sciences at Palo Alto. 1963 Business Week 13 July 61 There are many others..in the special groups or ‘think tanks’ that do analytical work for the armed forces, such as the Air Force's RAND Corp., the Navy's Operations Evaluation Group, the Army's Research Analysis Group, the Defense Dept.'s Institute for Defense Analysis. 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. 1968 Sunday Times 25 Feb. 10 The private research corporations, or ‘think tanks’ (in the current American terminology) which are paid, mostly by departments of Government, to think about problems. 1968 Economist 13 Apr. 29/3 ‘Think tank’ work is usually secret. 1969 Sunday Mail Mag. (Brisbane) 4 May 2/7 He's pretty busy as a ‘Business Doctor’ there runs a ‘think-tank’. 1970 Daily Tel. 22 Jan. 3/5 P & O, the world's largest shipping group, have formed a special company to exploit the inventions of their technical staff. A ‘think tank’ of 40 graduate engineers and naval architects will work on ideas and suggestions sent in by the sea-going and shore staffs. 1971 Ann. Reg. 1970 36 Lord Rothschild..described the task of the new organization (or, as it was nicknamed, the ‘think-tank’) as being to ‘weigh up the pros and cons and consequences’ of a proposed policy and to advise the Cabinet on it. 1973 Listener 26 Apr. 534/3 It has become the fashion among heads of governments and chiefs of state in the parliamentary democracies to equip themselves with a special staff of personal advisers a think-tank, an entourage.. whose views supplement and sometimes run counter to the processed findings of civil servants. 1976 H. WILSON Governance of Britain iv. 95 In addition to establishing the Central Policy Review Staff (the Think Tank),..he [sc. Edward Heath] initiated the practice of the Cabinet Office Units, which has been developed since. 1978 R. HILL Pinch of Snuff xiv. 146 He leaves the service, possibly under a cloud... The inference in the rugby club think-tank was that the cloud was sexual. 1981 Daily Tel. 15 Dec. 13/2 The Rand Corporation, a ‘think tank’ which undertakes various studies for the Pentagon, completed an analysis of all the Communist-bloc armies. Ibid. 29 Dec. 10/4 Lean times are looming for the private consulting firms, ‘think tanks’ and universities.

    3. A meeting or conference of experts, scholars, specialists, etc.

1976 T. SHARPE Wilt iv. 39 She had been accepted by people who flew to California or Tokyo to conferences and Think Tanks as casually as she took the bus to town. 1978 Washington Post 8 May A14/2 The meeting here, which one official called the NAACP's first ‘think tank’, was an effort to develop new positions for the association.

    Hence think-tanker, a member of a think-tank.

1971 New Scientist 2 Sept. 536/2 Throughout this century think-tankers have been confidently predicting the imminent exploitation..of the seas. 1975 ‘J. LYMINGTON’ Spider in Bath vii. 124 A brief report from our Think Tankers on hypnotising the village people.