The Collaborative International Dictionary of English v.0.48:

Mostly \Most"ly\, adv.
   For the greatest part; for the most part; chiefly; in the
   main.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mostly
    adv 1: in large part; mainly or chiefly; "These accounts are
           largely inactive" [syn: largely, mostly, for the
           most part]
    2: usually; as a rule; "by and large it doesn't rain much here"
       [syn: by and large, generally, more often than not,
       mostly]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "mostly":
   a fortiori, above all, all in all, all the more,
   all things considered, almost entirely, altogether, approximately,
   as a rule, as a whole, as an approximation, as per usual,
   as things go, as usual, at large, broadly, broadly speaking,
   by and large, chiefly, commonly, customarily, dominantly,
   effectually, especially, essentially, even, ever so, first of all,
   generally, generally speaking, habitually, in chief, in general,
   in the main, indeed, largely, mainly, more than ever, most often,
   naturally, never so, no end, normally, normatively, on balance,
   on the whole, ordinarily, overall, particularly, peculiarly,
   predominantly, prescriptively, prevailingly, primarily,
   principally, regularly, roughly, roughly speaking, routinely,
   speaking generally, still more, substantially, to be expected,
   usually, virtually, yea