Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

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