That is my sheet anchor—my 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 ship's 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.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894