The Collaborative International Dictionary of English v.0.48:

Chiefly \Chief"ly\, adv.
   1. In the first place; principally; pre["e]minently; above;
      especially.
      [1913 Webster]

            Search through this garden; leave unsearched no
            nook;
            But chiefly where those two fair creatures lodge.
                                                  --Milton.
      [1913 Webster]

   2. For the most part; mostly.
      [1913 Webster]

            Those parts of the kingdom where the . . . estates
            of the dissenters chiefly lay.        --Swift.
      [1913 Webster]

WordNet (r) 3.0 (2006):

chiefly
    adv 1: for the most part; "he is mainly interested in
           butterflies" [syn: chiefly, principally, primarily,
           mainly, in the main]

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "chiefly":
   a fortiori, ab initio, ab ovo, aborigine, 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 first, at large,
   at the start, before everything, broadly, broadly speaking,
   by and large, commonly, customarily, dominantly, effectually,
   especially, essentially, even, ever so, first, first and foremost,
   first of all, first thing, firstly, from the beginning, generally,
   generally speaking, habitually, in chief, in general,
   in particular, in the beginning, in the main, indeed, initially,
   largely, mainly, more than ever, most often, mostly, naturally,
   never so, no end, normally, normatively, on balance, on the whole,
   ordinarily, originally, overall, particularly, peculiarly,
   predominantly, prescriptively, prevailingly, primarily, primo,
   principally, regularly, roughly, roughly speaking, routinely,
   speaking generally, still more, substantially, to be expected,
   usually, virtually, yea