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):

excusable
    adj 1: capable of being overlooked [ant: inexcusable]
    2: easily excused or forgiven; "a venial error" [syn:
       excusable, forgivable, venial]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "excusable":
   admissible, allowable, condonable, defensible, dispensable,
   exemptible, expiable, forgivable, inoffensive, justifiable,
   legitimate, pardonable, reasonable, remissible, tenable,
   unobjectionable, venial, vindicable, warrantable