Back, v. i.
 1. To move or go backward; as, the horse refuses to back.
 2. Naut. To change from one quarter to another by a course opposite to that of the sun; -- used of the wind.
 3. Sporting To stand still behind another dog which has pointed; -- said of a dog. [Eng.]
 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.]
    Cleon at first . . . was willing to go; but, finding that he [Nicias] was in earnest, he tried to back out.   --Jowett (Thucyd. )


 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.
 2. To fill a cup or glass for drinking.
    Give me some wine; fill full.   --Shak.
 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.