The Collaborative International Dictionary
Unpenetrable
Unpenetrable \Un*pen"e*tra*ble\, a. Impenetrable.
Douglas Harper's Etymology Dictionary
unpenetrable
c.1400, from un- (1) "not" + penetrable (adj.). The usual word now is impenetrable.
Wiktionary
unpenetrable
a. impenetrable
Usage examples of "unpenetrable".
The Great King ruled from Phrygia in the west to the distant lands of the Hindu Kush, from fertile farmland to arid desert, from ice-covered forests to unpenetrable jungle.
Knee-length and paper-thin, they were nonetheless unpenetrable even if the boys should step on one of the needle-sharp ground thorns.