Source : Webster's Revised Unabridged Dictionary (1913)

Unimpeachable \Un`im*peach"a*ble\, a.
   Not impeachable; not to be called in question; exempt from
   liability to accusation; free from stain, guilt, or fault;
   irreproachable; blameless; as, an unimpeachable reputation;
   unimpeachable testimony. --Burke. --
   {Un`im*peach"a*ble*ness}, n. -- {Un`im*peach"a*bly}, adv.

Source : WordNet®

unimpeachable
     adj 1: beyond doubt or reproach; "an unimpeachable source"
     2: free of guilt; not subject to blame; "has lived a blameless
        life"; "of irreproachable character"; "an unimpeachable
        reputation" [syn: {blameless}, {inculpable}, {irreproachable}]
     3: completely acceptable; not open to exception or reproach;
        "two unexceptionable witnesses"; "a judge's ethics should
        be unexceptionable" [syn: {unexceptionable}]