What is "to back and fill"

Back \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 \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.