Sauter de bécarre en bémol (French), to jump from one subject to another without regard to pertinence; “Sauter du coq à l'ane,” 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.
Bémol is B mollis, soft (flat).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894