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]