To mull is to cogitate over something deeply and at length. To mull wine one warms it and adds 2 tablespoons mulling spices (cinnamon, cloves, allspice, etc.) and sugar. Judging from Oxford Dictionaries, the second usage of mull seems to be a century older. So... somehow the term for spicing Christmas wine became the term we use when deep in thought. Mulling involves a twenty-minute simmer, letting the spices mix with the fruity elements of the wine, producing a warm, delicious aroma. This simmer - not a hard, obvious boil - is a great metaphor for serious under-the-surface thought: it can't be (nor mustn't be) rushed; is quiet, not shouted; and the results are worth waiting for. As writers, we're mulling over a thousand things at once, envisioning worlds at a continuous simmer. The longer these worlds soak in the spices of inspiration, the sweeter they'll be.