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]

WordNet (r) 3.0 (2006):

unimpeachably
    adv 1: without question; "Fred Winter is unquestionably the
           jockey to follow"; "they hired unimpeachably first-rate
           faculty members" [syn: unquestionably, unimpeachably]