From WordNet (r) 2.0 (August 2003) [wn]:
mostly
adv 1: in large part; mainly or chiefly; "These accounts are
largely inactive" [syn: largely, {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]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Mostly \Most"ly\, adv.
For the greatest part; for the most part; chiefly; in the
main.
[1913 Webster]