The Collaborative International Dictionary of English v.0.48:

Back \Back\ (b[a^]k), n. [AS. b[ae]c, bac; akin to Icel., Sw., &
   LG. bak, Dan. bag; cf. OHG. bahho ham, Skr. bhaj to turn,
   OSlav. b[=e]g[u^] flight. Cf. Bacon.]
   1. In human beings, the hinder part of the body, extending
      from the neck to the end of the spine; in other animals,
      that part of the body which corresponds most nearly to
      such part of a human being; as, the back of a horse, fish,
      or lobster.
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   2. An extended upper part, as of a mountain or ridge.
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            [The mountains] their broad bare backs upheave
            Into the clouds.                      --Milton.
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   3. The outward or upper part of a thing, as opposed to the
      inner or lower part; as, the back of the hand, the back of
      the foot, the back of a hand rail.
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            Methought Love pitying me, when he saw this,
            Gave me your hands, the backs and palms to kiss.
                                                  --Donne.
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   4. The part opposed to the front; the hinder or rear part of
      a thing; as, the back of a book; the back of an army; the
      back of a chimney.
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   5. The part opposite to, or most remote from, that which
      fronts the speaker or actor; or the part out of sight, or
      not generally seen; as, the back of an island, of a hill,
      or of a village.
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   6. The part of a cutting tool on the opposite side from its
      edge; as, the back of a knife, or of a saw.
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   7. A support or resource in reserve.
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            This project
            Should have a back or second, that might hold,
            If this should blast in proof.        --Shak.
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   8. (Naut.) The keel and keelson of a ship.
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   9. (Mining) The upper part of a lode, or the roof of a
      horizontal underground passage.
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   10. A garment for the back; hence, clothing. [Obs.]
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             A bak to walken inne by daylight.    --Chaucer.
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   Behind one's back, when one is absent; without one's
      knowledge; as, to ridicule a person behind his back.

   Full back, Half back, Quarter back (Football), players
      stationed behind those in the front line.

   To be on one's back or To lie on one's back, to be
      helpless.

   To put one's back up or to get one's back up, to assume
      an attitude of obstinate resistance (from the action of a
      cat when attacked). [Colloq.]

   To see the back of, to get rid of.

   To turn the back, to go away; to flee.

   To turn the back on one, to forsake or neglect him.
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