A favourite topic that one frequently refers to or dwells on; a fixation.
The first things that were referred to as hobbies were in fact horses, of a breed that was popular in Ireland in the Middle Ages and is now extinct. The Scottish poet John Barbour referred to them as hobynis, in the narrative poem The Bruce, 1375. In Reliquiae Antiquae, a poetic work of Barbour's from around 1400 and republished in 1841, he referred to them again, this time with a little more context:
And one amang, an Iyrysch man,
Uppone his hoby swyftly ran,
English mummers, morris dance teams and minstrel groups began performing with characters (often children) dressed in wickerwork and cloth costumes, made to look like stylised horses - not altogether unlike the present-day pantomime horses. These 'hobby-horses', which took their name from the Irish breed, are still to be seen as part of the English folk tradition, notably at the annual 'Obby 'Oss festival, celebrated each May Day in Padstow, Cornwall. This custom dates back to at least the 16th century, when a payment for a performance by a hobby-horse was recorded in the Churchwarden's Accounts of St. Mary's Church, Reading, 1557:
Item, payed to the Mynstrels and the Hobby~horse on May Day, 3s.
As time went by, the name hobby-horse was given to numerous other things; for example,
A loose woman or strumpet:
William Shakespeare, Loves Labour's Lost, 1588 - "Cal'st thou my love Hobbi-horse?"
A child's nursery toy:
George Puttenham, The Arte of English Poesie, 1589 - "King Agesilaus, hauing a great sort of little children... tooke a little hobby horse of wood and bestrid it to keepe them in play."
A dance, similar to the stage antics of the mummers' horses:
Richard Lassels, The Voyage of Italy, circa 1668 - "Women like those that danced anciently the Hobby-horse in Country Mummings."
A favourite pursuit or pastime - later shortened of course just to hobby:
Sir Matthew Hale, Contemplations Moral and Divine, 1676 - "Almost every person hath some hobby horse or other wherein he prides himself."
A wooden horse fixed on a ‘merry-go-round’:
Gray's Letters and Poems, 1741 - "A Fair here is not a place where one eats gingerbread or rides upon hobby-horses."
A velocipede, on which the rider proceeded by pushing the ground with each foot alternately; also called a 'Dandy-horse':
The Gentleman's Magazine, February 1819 - "A machine denominated the Pedestrian Hobby-horse... has been introduced into this country by a tradesman in Long Acre."
It is the 'favourite pastime' version of the name, what we now call simply 'a hobby', that was adopted as a figurative expression meaning 'a fixation; a thing one keeps coming back to', i.e. similar to having a bee in one's bonnet.
So, a hobby is really a hobby-horse. If by any chance you occupy your spare time studying 13th century Irish livestock, your hobby-horse might just be a Hobby horse.