pet·rous (p?t?r?s)adj.1. Of, relating to, or resembling rock, especially in hardness; stony.2. Of or relating to the very dense, hard portion of the temporal bone that forms a protective case for the inner ear.[Middle English, from Old French petros, from Latin petr?sus, rocky, from petra, rock, from Greek petr?; see per- in Indo-European roots.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.petrous (?p?tr?s; ?pi?-) adj1. (Anatomy) anatomy denoting the dense part of the temporal bone that surrounds the inner ear2. rare like rock or stone[C16: from Latin petr?sus full of rocks]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014pet?rous (?p? tr?s, ?pi-) adj. like stone, esp. in hardness; stony; rocky. [1350?1400; Middle English (