The Collaborative International Dictionary of English v.0.48:

Bottom \Bot"tom\ (b[o^]t"t[u^]m), n. [OE. botum, botme, AS.
   botm; akin to OS. bodom, D. bodem, OHG. podam, G. boden,
   Icel. botn, Sw. botten, Dan. bund (for budn), L. fundus (for
   fudnus), Gr. pyqmh`n (for fyqmh`n), Skr. budhna (for
   bhudhna), and Ir. bonn sole of the foot, W. bon stem, base.
   [root]257. Cf. 4th Found, Fund, n.]
   1. The lowest part of anything; the foot; as, the bottom of a
      tree or well; the bottom of a hill, a lane, or a page.
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            Or dive into the bottom of the deep.  --Shak.
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   2. The part of anything which is beneath the contents and
      supports them, as the part of a chair on which a person
      sits, the circular base or lower head of a cask or tub, or
      the plank floor of a ship's hold; the under surface.
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            Barrels with the bottom knocked out.  --Macaulay.
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            No two chairs were alike; such high backs and low
            backs and leather bottoms and worsted bottoms. --W.
                                                  Irving.
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   3. That upon which anything rests or is founded, in a literal
      or a figurative sense; foundation; groundwork.
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   4. The bed of a body of water, as of a river, lake, sea.
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   5. The fundament; the buttocks.
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   6. An abyss. [Obs.] --Dryden.
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   7. Low land formed by alluvial deposits along a river;
      low-lying ground; a dale; a valley. "The bottoms and the
      high grounds." --Stoddard.
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   8. (Naut.) The part of a ship which is ordinarily under
      water; hence, the vessel itself; a ship.
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            My ventures are not in one bottom trusted. --Shak.
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            Not to sell the teas, but to return them to London
            in the
            same bottoms in which they were shipped. --Bancroft.
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   Full bottom, a hull of such shape as permits carrying a
      large amount of merchandise.
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   9. Power of endurance; as, a horse of a good bottom.
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   10. Dregs or grounds; lees; sediment. --Johnson.
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   At bottom, At the bottom, at the foundation or basis; in
      reality. "He was at the bottom a good man." --J. F.
      Cooper.

   To be at the bottom of, to be the cause or originator of;
      to be the source of. [Usually in an opprobrious sense.]
      --J. H. Newman.
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            He was at the bottom of many excellent counsels.
                                                  --Addison.
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   To go to the bottom, to sink; esp. to be wrecked.

   To touch bottom, to reach the lowest point; to find
      something on which to rest.
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WordNet (r) 3.0 (2006):

at bottom
    adv 1: in reality; "she is very kind at heart" [syn: at heart,
           at bottom, deep down, inside, in spite of
           appearance]