Material or masonry used to support a structure, such as a wall.
A support or foundation. Often used in the plural.
Informal The human legs. Often used in the plural.
un·der·pin (ŭńdər-pĭń)
transitive verb: -pinned, -pin·ning, -pins.
To support from below, as with props, girders, or masonry.
To give support or substance to: “the public awareness that must underpin a sustained and concerted development effort for Africa and its youth” (Barber B. Conable)
The American Heritage® Dictionary of the English Language, Fourth Edition