From WordNet (r) 3.0 (2006) [wn]:
underside
n 1: the lower side of anything [syn: bottom, underside,
undersurface]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Underside \Un"der*side`\, n.
The lower or lowest side of anything. --Paley.
[1913 Webster]