From WordNet (r) 3.0 (2006) [wn]:
mostly
adv 1: in large part; mainly or chiefly; "These accounts are
largely inactive" [syn: largely, mostly, {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,
mostly]
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]