on (one's) hobby-horse
Frequently or incessantly talking about a subject, topic, or issue in which one is excessively interested. "Hobby-horse" 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.
Farlex Dictionary of Idioms. © 2024 Farlex, Inc, all rights reserved.