UNIMPEACHABLY
unquestionably, unimpeachably
(adverb) without question; “Fred Winter is unquestionably the jockey to follow”; “they hired unimpeachably first-rate faculty members”
Source: WordNet® 3.1
Etymology
Adverb
unimpeachably (comparative more unimpeachably, superlative most unimpeachably)
In an unimpeachable manner.
Source: Wiktionary
UNIMPEACHABLE
Un`im*peach"a*ble, a.
Definition: 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: Webster’s Unabridged Dictionary 1913 Edition