hob ‧ by ‧ horse /ˈhɒbihɔːs $ ˈhɑːbihɔːrs/ BrE AmE noun [countable]
[ Date: 1500-1600 ; Origin: hobby 'small horse' (14-19 centuries) (probably from Hobbin , a man's name, from Robin ) + horse ]
1 . a subject that someone has strong opinions about and that they talk about too much:
I’m afraid safe driving isn’t Jaqui’s only hobbyhorse.
be on your hobbyhorse
Vicky was on her hobbyhorse again.
2 . an old-fashioned toy made of a horse’s head on a stick