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