That was my sheet anchor—i.e. my best hope, my last refuge. The sheet anchor is the largest anchor of a ship, which, in stress of weather, is the sailor's chief dependence. The word sheet is a corruption of the word shote (thrown out), meaning the anchor “thrown out” in foul weather. The Greeks and Romans said, “my sacred anchor,” because the sheet anchor was always dedicated to some god. The Anchor, in Christian art, is given to Clement of Rome and Nicolas of Bari. Pope Clement, in A.D. 80, was bound to an anchor and cast into the sea. Nicolas of Bari is the patron saint of sailors. The anchor is apeak, that is, the cable of the anchor is so tight that the ship is drawn completely over it. (See Bower Anchor, Sheet Anchor.)
The Anchor comes home, the anchor has been dragged from its hold. Figuratively, the enterprise has failed, notwithstanding the precautions employed.
To weigh anchor to haul in the anchor, that the ship may sail away from its mooring. Figuratively, to begin an enterprise which has hung on hand.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894