The Collaborative International Dictionary of English v.0.48:

Mainly \Main"ly\, adv. [From main strong. See Main strength.]
   Very strongly; mightily; to a great degree. [Obs.] --Bacon.
   Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mainly \Main"ly\, adv. [From main principal, chief.]
   Principally; chiefly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mainly
    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:

83 Moby Thesaurus words for "mainly":
   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 bottom, at first,
   at large, at the start, before everything, broadly,
   broadly speaking, by and large, chiefly, commonly, customarily,
   dominantly, effectively, 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 the beginning, in the main, indeed, initially, largely,
   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