be on your hobbyhorse
if someone is on their hobbyhorse, they are talking about a subject which they think is interesting and important, and which they talk about at any time that they can, even if other people are not interested

[TahlilGaran] Dictionary of Contemporary English ▲

on your hobbyhorse
on (your) hobbyhorse
if someone is on their hobbyhorse, they are talking about a subject which they think is interesting and important, and which they talk about at any time that they can, even if other people are not interested.
Don't mention tax, or Bernard'll get on his hobbyhorse again.

[TahlilGaran] English Idioms Dictionary ▲