hobbyhorse
noun [ C ] uk/ˈhɒb.i.hɔːs/ us/ˈhɑː.bi.hɔːrs/
(TOY)
（儿童当马骑的）马头杆，竹马
a toy made from a long stick, with a shape like a horse's head at one end, that a child can pretend to ride
(SUBJECT)
反复提到的话题
a subject that someone often talks about, usually for a long time
Don't mention tax or Bernard'll get on his hobbyhorse again. 别提税收，否则伯纳德又要反复唠叨这个话题了。