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unpassable

adjective un·pass·able \ ˌən-ˈpa-sə-bəl \

Definition of unpassable

:incapable of being traveled, traveled through, or crossed :impassible

… a continent of land of at least 1800 miles, in which journey we had … unpassable deserts to go over …
—Daniel Defoe

First Known Use of unpassable

circa 1525

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