E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Sauter de bécarre en bémol (French), to jump from one subject to another without regard to pertinence; Sauter du coq à lane, from Genesis to Revelation. Literally, to jump from sharps to flats. Becarre is the Latin B quadratum or B quarré. In old musical notation B sharp was expressed by a square B, and B flat by a round B.