Hobbyhorse
Definition 1 of 2

Contextual Use
When referring to the literal toy, it's best to use it in contexts involving children or play.

Many children love playing with their hobbyhorses in the park.
Metaphorical Use
The metaphorical sense is often used to describe someone's pet subject or passion.

He always gets on his hobbyhorse about politics during dinner.
Compare with
hobbyhorseobsessioninterestpastimepassion
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