an under or lower side.

1670–80; under- + side1

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World English Dictionary
underside (ˈʌndəˌsaɪd)
the bottom or lower surface

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

c.1680, from under + side (n.). Cf. Du. onderzijde, Dan. underside, Ger. unterseite.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
But the air on the bottom still strikes the underside of the wing, pushes it up
  into the partial vacuum and then bounces down.
The light then bounces off the bottom mirror, and then bounces again off the
  underside of the top mirror.
The underside of the leather, which is suede, should be the same color as the
  finished side.
If so, the plastron would have shielded the turtle's underside from predators
  approaching from below.
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