hobby horse

Từ điển Oxford Advanced Learner 8th

ˈhobby horse [hobby horse] BrE NAmE noun
1. (sometimes disapproving)a subject that sb feels strongly about and likes to talk about
•to get on your hobby horse (= talk about your favourite subject)
2. a toy made from a long stick that has a horse's head at one end. Children pretend to ride on it.