If you say that something is unexcusable, you are emphasizing that it cannot be
justified or tolerated because it is
extremely bad.
The earliest known use of the adjective unexcusable is
in the Middle English period (1150—1500). OED's earliest evidence for unexcusable is from around 1384, in Bible (Wycliffite, early version).
unexcusable is formed within English, by derivation.
Etymons: un- prefix1 1b, excusable adj.