Source : Webster's Revised Unabridged Dictionary (1913)

Hobby \Hob"by\, Hobbyhorse \Hob"by*horse`\, 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.

   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.

Source : WordNet®

hobbyhorse
     n 1: a topic to which one constantly reverts; "don't get him
          started on his hobbyhorse"
     2: a child's plaything consisting of an imitation horse mounted
        on rockers; the child straddles it and pretends to ride
        [syn: {hobby}, {rocking horse}]