subs. phr. (colloquial).—A rough-and-ready way: practical rather than exactly scientific (GROSE).

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  1809.  SYDNEY SMITH, To Francis Jeffrey, 3 Sept. We’ll settle men and things by RULE OF THUMB.

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  1864.  Daily Review, 17 Oct. The result, we trust, will exemplify the value of Science versus RULE OF THUMB in politics.

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