underside

from WordNet (r) 3.0 (2006)

underside
    n 1: the lower side of anything [syn: {bottom}, {underside},
         {undersurface}]

from The Collaborative International Dictionary of English v.0.48

Underside \Un"der*side`\, n.
   The lower or lowest side of anything. --Paley.
   [1913 Webster]

from 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

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