hobby

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.
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2. A stick, often with the head or figure of a horse, on which boys make believe to ride. [ Usually under the form
{hobbyhorse}.]
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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}.]
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Not one of them has any hobbyhorse, to use the phrase of Sterne. --Macaulay.
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Source: The Collaborative International Dictionary of English