Green’s Dictionary of Slang

hobby horse n.

1. [late 16C–early 17C] a fool, a jester [SE hobby horse, the performer, in a morris dance, who manipulates, with much capering, the wicker horse that is part of the trad. ‘cast’].

2. [late 16C–18C] (also she-hobby) a prostitute, a promiscuous woman; a mistress [SE hobby horse, a small horse; she can be ‘ridden’ by all and sundry].