Late Middle English hobyn, hoby, from pet forms of the given name Robin. Originally in sense 2 (compare with dobbin), it later came to denote a toy horse or hobby horse, hence 'an activity done for pleasure'.

• In medieval times men and boys given the name Robin were sometimes known as Hobin or Hobby, in much the same way that today they might be called Bob or Bobby. This became a pet term for a pony, just as Dobbin—also from Robin—was used for a carthorse. This gave us the hobby horse (mid 16th century), a figure of a horse made of wickerwork and worn over the head in a Morris dance or pantomime. Later it became a stick with a horse's head, for a child to ride when playing. The connection with pleasure or play led to the use of hobby horse for what we now call a hobby. Since the early 19th century hobby has taken over this sense and hobby horse now usually means ‘a preoccupation or favourite topic’. The hob in hobgoblin (mid 16th century) is also from a pet form of Robin or Robert.

Late Middle English: from Old French hobet, diminutive of hobe 'falcon'.