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.