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.
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