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Webster 1913 Edition

adv.

In an irretrievable manner.

adv.

Irreparably; irrecoverably; in a manner not to be regained.

irretrievably ‎(comparative more irretrievably, superlative most irretrievably)

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