a hobby horse

hobby-horse

A subject, topic, or issue about which one frequently or incessantly talks, expounds, or complains. Ah, here we go again. Once grandpa gets on his hobby-horse about the government, there's no stopping him!

a ˈhobby horse

a subject that somebody feels strongly about and likes to talk about: Sorry, I’m starting to get on my hobby horse! (= talk about my favourite subject).See also: hobby, horse