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):

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, unimpeachable]
    3: completely acceptable; not open to exception or reproach;
       "two unexceptionable witnesses"; "a judge's ethics should be
       unexceptionable" [syn: unexceptionable, unimpeachable]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "unimpeachable":
   Christian, above suspicion, absolute, actual,
   admitting no question, believable, blameless, clean, colorable,
   conceivable, confirmable, credible, creditable, decent,
   demonstrable, demonstratable, erect, estimable, ethical, factual,
   fair, fiduciary, full of integrity, good, high-minded,
   high-principled, highly respectable, historical, honest, honorable,
   immaculate, inappealable, incontestable, incontrovertible,
   inculpable, indisputable, indubitable, inviolate, irrefragable,
   irrefutable, irreprehensible, irreproachable, irreprovable, just,
   law-abiding, law-loving, law-revering, manly, moral, noble,
   plausible, principled, provable, pure, real, reliable, reputable,
   respectable, right, right-minded, righteous, self-evident,
   spotless, stainless, sterling, tenable, testable, true-dealing,
   true-devoted, true-disposing, true-souled, true-spirited,
   truehearted, trustworthy, trusty, unanswerable, unarraignable,
   unblamable, unblameworthy, unblemished, uncensurable, unconfutable,
   uncorrupt, uncorrupted, undefiled, undeniable, unexceptionable,
   unindictable, unobjectionable, unquestionable, unrefutable,
   unspotted, unstained, unsullied, untarnished, upright, uprighteous,
   upstanding, verifiable, virtuous, worthy, worthy of faith,
   yeomanly