hobby-horse
A subject, topic, or issue about which one frequently or incessantly talks, expounds, or complains. The term can also be spelled as a single word. Ah, here we go again. Once Grandpa gets on his hobby-horse about the government, there's no stopping him! Uh oh, sounds like Janet is on her hobbyhorse again about the need for healthcare reform. If Joe gets on his hobby-horse about public safety, the Dean of Students will be listening to that for a long time.
Farlex Dictionary of Idioms. © 2024 Farlex, Inc, all rights reserved.