a hobby horse

hobby-horse

A subject, topic, or issue about which one frequently or incessantly talks, expounds, or complains. The term can also be spelled as a single word. Ah, here we go again. Once grandpa gets on his hobby-horse about the government, there's no stopping him! Uh oh, sounds like Janet is on her hobbyhorse again about the need for healthcare reform.
Farlex Dictionary of Idioms.

a ˈhobby horse

a subject that somebody feels strongly about and likes to talk about: Sorry, I’m starting to get on my hobby horse! (= talk about my favourite subject).
See also: hobby, horse
Farlex Partner Idioms Dictionary
See also:
  • hobby-horse
  • on (one's) hobby-horse
  • ride (one's) hobby-horse
  • get on (one's) hobby-horse
  • hobby
  • hobbyhorse
  • cry-baby
  • see the last of
  • see the last of (someone or something)
  • see the end of (someone or something)
References in periodicals archive
Crude in nature, this toy mimicked the back of a horse better than a hobby horse.
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There are numerous videos on YouTube about how to make a hobby horse, and the components are easy to find: a broomstick, fabric, fur, wool, stuffing and decorations - which are obviously up to each creator.
The competition in Finland is real, but the horses -- not so much
When the son emerges it is to ask grandma if she remembers making him a hobby horse when he was small.
Inside out: Down in the mouth, something wobbled; THE FAMILY WAY
* A hobby horse pattern (Cut out the hobby horse shown here.
Holiday hobby horse
For example, George Tollet, from his retirement at Betley Hall in Staffordshire, contributed to the 1778 Johnson-Steevens edition a long note describing stained glass figures of morris dancers, Maid Marion, a hobby horse and a Maypole.
Shakespeare's Midwives: Some Neglected Shakespeareans