noun the foundation, ·esp. of a frame house.
2. underpinning ·p.pr. & ·vb.n. of underpin.
3. underpinning ·noun the act of one who underpins; the act of supporting by stones, masonry, or the like.
4. underpinning ·noun that by which a building is underpinned; the material and construction used for support, introduced beneath a wall already constructed.