Definify.com
Webster 1913 Edition
Underpinning
Un′der-pinˊning
,Noun.
1.
The act of one who underpins; the act of supporting by stones, masonry, or the like.
2.
(Arch.)
(a)
That by which a building is underpinned; the material and construction used for support, introduced beneath a wall already constructed.
(b)
The foundation, esp. of a frame house.
[Local, U. S.]
Webster 1828 Edition
Underpinning
UNDERPIN'NING
,ppr.
UNDERPIN'NING
, n.1.
The act of laying stones under sills.2.
The stones on which a building immediately rests.