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Webster 1913 Edition

Adj.

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.

–

Unˊim-peach′a-ble-ness

Noun.

–

Unˊim-peach′a-bly

adv.

, a.

That cannot be impeached; that cannot be accused; free from stain, guilt or fault; as an unimpeachable reputation.

That cannot be called in question; as an unimpeachable claim or testimony.

unimpeachable ‎(comparative more unimpeachable, superlative most unimpeachable)

Not able to be impeached or reproached.
a gentleman of unimpeachable integrity
Blameless.
Beyond doubt.
an unimpeachable alibi