___ mater (brain protector)

DURA

This tough membrane lies just beneath the skull and is the outermost of the three layers covering the brain and spinal cord.

This clue is looking for a Latin term meaning "hard mother," which refers to the dura mater—the tough outer membrane that shields the brain and spinal cord. The blank hints at a three-letter prefix, and "mater" is Latin for "mother," so the answer fits both the definition and the wordplay. It’s a common term in anatomy but might sound unfamiliar if you’re not in the medical field. Think of it like the brain’s built-in helmet.