Mr. Lowell says, “We have heard mulling used for stirring, bustling, sometimes in an underhand way. It is a metaphor probably from mulling wine.”

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18[?].  There has been a pretty considerable mullin going on among the doctors ever sen the quack medicine came out.—‘Margaret’ (Bartlett). Here it means consulting.

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1857.  “And what do you do with them [your troubles]?” “Let ’em mull.”—J. G. Holland, ‘The Bay-Path,’ p. 200. [Let ’em settle themselves.]

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1897.  The question is, what kind of a boy was he? I’ve been mullin’ [ruminating] over that consid’able.—W. D. Howells, ‘Landlord at Lion’s Head,’ ch. xxxviii.

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