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®

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