underpinning

noun3

1. •
   A support or foundation, especially as a structure of masonry that supports a wall.

   • This house's underpinning is not up to standard.
2. •
   (figurative) A basis for something.

   • I would like to know more about the underpinning of this political theory.
3. •
   (humorous, figurative) The lower body or legs.