Mr. Lowell says, We have heard mulling used for stirring, bustling, sometimes in an underhand way. It is a metaphor probably from mulling wine.
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.
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.]
1897. The question is, what kind of a boy was he? Ive been mullin [ruminating] over that considable.W. D. Howells, Landlord at Lions Head, ch. xxxviii.