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The Collaborative International Dictionary
Unexcusable

Unexcusable \Un`ex*cus"a*ble\, a. Inexcusable.
--Hayward. -- Un`ex*cus"a*ble*ness, n.

Douglas Harper's Etymology Dictionary
unexcusable

late 14c., from un- (1) "not" + excusable. The usual word is inexcusable. Related: Unexcusably.

Wiktionary
unexcusable

a. inexcusable