unpenetrable

adjective

1. capable of being penetrated.

adj.c.1400, from un- (1) “not” + penetrable. The usual word now is impenetrable. adj.early 15c., “penetrating,” from Latin penetrabilis “penetrable, vulnerable,” from penetrare (see penetrate). Meaning “capable of being penetrated” is attested from 1530s; figurative use by 1590s. Related: Penetrability.