The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]