HOBBYHORSE
hobby, hobbyhorse, rocking horse
(noun) a child’s plaything consisting of an imitation horse mounted on rockers; the child straddles it and pretends to ride
hobbyhorse
(noun) a topic to which one constantly reverts; “don’t get him started on his hobbyhorse”
Source: WordNet® 3.1
Noun
hobbyhorse (plural hobbyhorses)
Alternative spelling of hobby horse
Source: Wiktionary
Hob"by, Hob"by*horse`, n. Etym: [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: Webster’s Unabridged Dictionary 1913 Edition