E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
That is my sheet anchormy chief stay, my chief dependence. The sheet anchor is the largest and heaviest of all. The word is a corruption of Shote-anchor, the anchor shot or thrown out in stress of weather. Many ships carry more than one sheet-anchor outside the ships waist.
The surgeon no longer bleeds. If you ask him why this neglect of what was once considered the sheet anchor of practice in certain diseases? he will The Times.