underpinnings

noun

1. a system of supports beneath a wall or the like.
2. Often underpinnings. a foundation or basis: to uncover the emotional underpinnings of an illness.
3. underpinnings, Informal.
   1. underwear, especially women’s underwear.
   2. the legs.

pl n

1. any supporting structure or system

noun

1. a structure of masonry, concrete, etc, placed beneath a wall to provide support

n.late 15c., “action of supporting or strengthening from beneath,” from under + present participle of pin (v.). Figurative sense of “prop, support” is recorded from 1580s.