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Webster 1913 Edition

adv.

In an irreproachable manner; blamelessly.

adv.

In a manner not to deserve reproach; blamelessly; as deportment irreproachably upright.

irreproachably ‎(comparative more irreproachably, superlative most irreproachably)

irreproachably in The Century Dictionary, The Century Co., New York, 1911
irreproachably in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913