hobby-horse
n
1 long stick with a horse's head, used as a toy 馬頭桿子(玩具); 竹馬.
2 subject that a person likes to discuss; favourite topic of conversation (討論、 交談中)喜愛的話題
You've got me onto (ie talking about) one of my favourite hobby-horses. 你把我引到我喜愛的一個話題上來了.