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Webster 1913 Edition

adv.

In an irresistible manner.

adv.

With a power that cannot be successfully resisted or opposed.

irresistibly ‎(comparative more irresistibly, superlative most irresistibly)

in an irresistible manner

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