From Longman Dictionary of Contemporary Englishhobbyhorsehob‧by‧horse /ˈhɒbihɔːs $ ˈhɑːbihɔːrs/ noun [countable] 1 TALK/MAKE A SPEECHa subject that someone has strong opinions about and that they talk about too much I’m afraid safe driving isn’t Harry’s only hobbyhorse.be on your hobbyhorse Vicky was on her hobbyhorse again.2 DHTan old-fashioned toy made of a horse’s head on a stick
Examples from the Corpus
hobbyhorse• Another excellent chapter deals with the greenhouse effect and ozone depletion, which Brennan admits is a personal hobbyhorse.Origin hobbyhorse (1500-1600) hobby “small horse” ((14-19 centuries)) (probably from Hobbin, a man's name, from Robin) + horse 