hobby.

From Middle English hoby (“small horse, pony”), a variant or alteration of hobyn (“a small ambling horse”), of uncertain and disputed origin. According to one theory the term is derived from Hobin, a pet name of Robert or Robin, at the time frequently used as a proper name for a workhorse (compare dobbin). Alternatively, possibly from Anglo-Norman hobyn, Old French hobin, obin ("a nag, hobby"; > Modern French aubin, Italian ubino): either derived from Old French hober, ober (“to stir, move”), from Old Dutch *hobbon ("to toss, move up and down"; whence Middle Dutch hobbelen (“to bump along”)); or possibly from North Germanic origin related to Danish hoppe (“a mare”), Old Swedish hoppa (“a young mare”), North Frisian hoppe (“horse”); both routes ultimately deriving from Proto-Germanic *huppōną (“to bob, hop”), from Proto-Indo-European *kewb- (“to bend; a bend, joint”). More at hop, hobble. The modern sense of "pastime, activity of interest" is a clipping of hobby horse, which had 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".

From Middle English hoby, hobeye, from Old French hobé, hobei, hobet, from Medieval Latin hopētus, diminutive of harpe.