also hobby-horse, 1550s, "mock horse used in the morris-dance;" 1580s, "child's toy riding horse," from hobby (n.) + horse (n.). Transferred sense of "favorite pastime or avocation" first recorded 1670s (shortened to hobby by 1816). The connecting notion being "activity that doesn't go anywhere."
The hobbyhorse originally was a "Tourney Horse," a wooden or basketwork frame worn around the waist and held on with shoulder straps, with a fake tail and horse head attached, so the wearer appears to be riding a horse. These were part of church and civic celebrations at Midsummer and New Year's throughout England.
c. 1400, hobi, "small, active horse," short for hobyn (mid-14c.; late 13c. in Anglo-Latin), probably originally a proper name for a horse (compare dobbin), a diminutive of Robert or Robin. Old French hobi, hobin, once considered possible sources, now are held to be borrowings from English.
The modern sense of "a favorite pursuit, object, or topic" is from 1816, a shortening of hobbyhorse (q.v.) in this sense, which is attested from 1670s. Earlier it meant "a wooden or wickerwork figure of a horse," as a child's toy or a costume in the morris-dance, the connecting notion being "activity that doesn't go anywhere." Hobby as a shortening of hobbyhorse also was used in the "morris horse" sense (1760) and the "child's toy horse" sense (1680s).
1920, from French dada "hobbyhorse," child's nonsense word, selected 1916 by Romanian poet Tristan Tzara (1896-1963), leader of the movement, for its resemblance to meaningless babble.
Freedom: DADA DADA DADA, the howl of clashing colors, the intertwining of all contradictions, grotesqueries, trivialities: LIFE. [T. Tzara, "Dada Manifesto," 1918]
Related: Dadaist; Dadaism.