E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
That was my sheet anchori.e. my best hope, my last refuge. The sheet anchor is the largest anchor of a ship, which, in stress of weather, is the sailors 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.