To back an anchor

to increase the holding power by laying down a small anchor ahead of that by which the ship rides, with the cable fastened to the crown of the latter to prevent its coming home.
(Naut.) to lay down a small anchor ahead of a large one, the cable of the small one being fastened to the crown of the large one.

See also: Anchor, Back

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.