E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Catch a Crab (To).
In rowing, is to be struck with the handle of ones oar; to fall backwards. This occurs when the rower leaves his oar too long in the water before repeating the stroke. In Italian granchio is a crab, and pigliar il granchio is to catch a crab, or a Tartar.