A hobbyhorse is a toy made from a stick with a horse's head attached to one end. Children “ride” it by straddling the stick and pretending to gallop around, holding the horse's head out in front. Before bicycles and scooters, hobbyhorses were among the most popular toys for active play.

The word has taken on a second meaning that's quite different. Someone's hobbyhorse is a favorite topic they bring up constantly, especially when others wish they'd talk about something else. If your uncle can't stop talking about vintage trains at every family dinner, you might say that vintage trains are his hobbyhorse. The image works because just as a child might ride a toy horse everywhere, a person “rides” their favorite subject into every conversation, sometimes whether it fits or not.

When someone “gets on their hobbyhorse,” it means they've started talking enthusiastically about their pet subject again. The term suggests gentle teasing rather than real criticism. Everyone has topics they find endlessly fascinating, even if others don't share that passion quite as strongly.