The Collaborative International Dictionary of English v.0.48:

Excusable \Ex*cus"a*ble\a. [L. excusabilis: cf. F. excusable.
   See Excuse.]
   That may be excused, forgiven, justified, or acquitted of
   blame; pardonable; as, the man is excusable; an excusable
   action. -- Ex*cus"a*ble*ness, n. -- Ex*cus"a*bly, adv.
   [1913 Webster]

         The excusableness of my dissatisfaction. --Boyle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

excusably
    adv 1: in an excusable manner or to an excusable degree; "he was
           excusably late" [syn: excusably, forgivably,
           pardonably] [ant: inexcusably, unforgivably,
           unpardonably]