nether region

noun3

1. •
   Hell; a realm beneath the surface of the earth conceived as the abode of the souls of the dead and, sometimes, as the abode of demons or evil spirits.

   • At this instant old Roger Chillingworth thrust himself through the crowd—or, perhaps, so dark, disturbed, and evil was his look, he rose up out of some nether region. — 1850
2. •
   (humorous, euphemistic) The private parts between the legs; the groin or buttocks.

   • The rash of celebrities flashing their nether regions worries Peter Post, director of the Emily Post Institute of etiquette and manners. — 2006
3. •
   A place which is subterranean or enclosed beneath a surface, especially one which is dark, dank, or otherwise inhospitable.

   • But the steward of those nether regions marked him, by the electric lamps, as a lurking passenger to be watched; and wondered who, at that depth in the ship, could be carrying valuables. — 1918