Hobby
late Middle English hobyn, hoby, from pet forms of the given name Robin . Originally in hobby1 (sense 2) (compare with dobbin), it later came to denote a toy horse or hobby horse, hence ‘an activity done for pleasure’.
wiktionary
Shortened from hobby-horse, from Middle English hoby, hobyn, hobin(“small horse, pony”), from Old French hobi, *haubi, haubby, hobin("a nag, hobby"; > Modern French aubin, Italian ubino), of Germanic origin: from Old French hober, ober(“to stir, move”), from Old Dutch hobben(“to toss, move up and down”); or from North Germanic origin related to Danish hoppe(“a mare”), Old Swedish hoppa(“a young mare”), North Frisian hoppe(“horse”); both ultimately from Proto-Germanic *huppōną(“to hop”), from Proto-Indo-European *kewb-(“to bend; a bend, joint”). More at hop, hobble.
The meaning of hobby-horse shifted from "small horse, pony" to "child's toy riding horse" to "favorite pastime or avocation" with the connecting notion being "activity that doesn't go anywhere". Possibly originally from a proper name for a horse, a diminutive of Robert or Robin (compare dobbin).
From Old French hobet, from Medieval Latin hopētus, diminutive of harpe.
etymonline
hobby (n.)
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).
