hobbyhorse

{ } n. [ OE. hobin a nag, OF. hobin hobby; cf. hober to stir, move; prob. of German or Scand. origin; cf. Dan. hoppe a mare, dial. Sw. hoppa; perh. akin to E. hop to jump. ] 1. A strong, active horse, of a middle size, said to have been originally from Ireland; an ambling nag. Johnson. [ 1913 Webster ]

2. A stick, often with the head or figure of a horse, on which boys make believe to ride. [ Usually under the form hobbyhorse. ] [ 1913 Webster ]

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. ] [ 1913 Webster ]

Not one of them has any hobbyhorse, to use the phrase of Sterne. Macaulay. [ 1913 Webster ]