When the word hobby was first borrowed into English around the turn of the fifteenth century, it meant "small horse"! This has a very interesting history. By 1689, the word got a new definition of hobby-horse, a term that's still in use to refer to those children's toys with stuffed horses' heads on sticks. The playthings were regarded to be very childish, so to hobbyhorse became a verb meaning "engage in a fanciful pursuit". Sometime in the early nineteenth century, the -horse part was dropped and the old definition of hobby grew archaic, leaving us with the current state of the word. Going backwards, hobby used to be spelled hobyn or hobin and was likely a proper name for a horse, which the Oxford English Dictionary suggest ultimately is some kind of diminutive for the name Robert or Robin.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.