1540s, from Middle French petreux, from Latin petrosus "stony," from petra "rock," from Greek petra "rock, cliff, ledge, shelf of rock, rocky ridge," of uncertain origin. Possibly from PIE root *per- "to lead, pass over," if the original meaning is "bedrock" and the notion is "what one comes through to" [Watkins].
petrous pet·rous (pět'rəs)
Of stony hardness.
Of or relating to the dense hard portion of the temporal bone that forms a protective case for the inner ear.