The Collaborative International Dictionary of English v.0.48:

Back \Back\, v. i.
   1. To move or go backward; as, the horse refuses to back.
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   2. (Naut.) To change from one quarter to another by a course
      opposite to that of the sun; -- used of the wind.
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   3. (Sporting) To stand still behind another dog which has
      pointed; -- said of a dog. [Eng.]
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   To back and fill, to manage the sails of a ship so that the
      wind strikes them alternately in front and behind, in
      order to keep the ship in the middle of a river or channel
      while the current or tide carries the vessel against the
      wind. Hence: (Fig.) To take opposite positions
      alternately; to assert and deny. [Colloq.]

   To back out, To back down, to retreat or withdraw from a
      promise, engagement, or contest; to recede. [Colloq.]
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            Cleon at first . . . was willing to go; but, finding
            that he [Nicias] was in earnest, he tried to back
            out.                                  --Jowett
                                                  (Thucyd. )
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The Collaborative International Dictionary of English v.0.48:

Fill \Fill\, v. i.
   1. To become full; to have the whole capacity occupied; to
      have an abundant supply; to be satiated; as, corn fills
      well in a warm season; the sail fills with the wind.
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   2. To fill a cup or glass for drinking.
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            Give me some wine; fill full.         --Shak.
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   To back and fill. See under Back, v. i.

   To fill up, to grow or become quite full; as, the channel
      of the river fills up with sand.
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