[1] : The Collaborative International Dictionary of English v.0.48

underside \un"derside`\, n.
   the lower or lowest side of anything. --paley.
   1913 webster

[2] : WordNet (r) 2.0

underside
     n : the lower side of anything syn: bottom, undersurface
see also:
bottom undersurface 

[3] : Moby Thesaurus II by Grady Ward, 1.0

21 moby thesaurus words for "underside":
   bed, bedrock, belly, bottom, bottom side, breech, buttocks,
   downside, fundament, hardpan, lower side, lowest layer,
   lowest level, nether side, nethermost level, rock bottom, sole,
   substratum, underbelly, underlayer, underneath

Underside >> U Words
Underside, definition of term: Underside
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