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!
get on (one's) hobby-horse
To frequently or incessantly talk or complain about a subject, topic, or issue in which one is excessively interested. Ah, here we go again. Once grandpa gets on his hobby-horse about the government, there's no stopping him!
ride (one's) hobby-horse
To frequently or incessantly talk or complain about a subject, topic, or issue in which one is excessively interested. Ah, here we go again. Once grandpa starts riding his hobby-horse about the government, there's no stopping him!