Hobby; Hobbyhorse

Part of speech: noun (n.)

1. A strong, active horse, of a middle size, said to have been originally from Ireland; an ambling nag. Johnson.

2. A stick, often with the head or figure of a horse, on which boys make believe to ride. [ Usually under the form hobbyhorse.]

3. A subject or plan upon which one is constantly setting off; a favorite and ever-recurring theme of discourse, thought, or effort; that which occupies one's attention unduly, or to the weariness of others; a ruling passion. [Usually under the form hobby.] Not one of them has any hobbyhorse, to use the phrase of Sterne. Macaulay.