What's the "een" in Halloween?
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.