When a fashionable jockey takes the already booked mount of a lesser rider"
I learned that at the website for The National Horseracing Museum (at Newmarket, Suffolk, UK). It's on the glossary page, along with other spiffy racetalk like act in the dip, bumping and boring, and tap root mare.
There's also a page full of games and quizzes, and an archive chock full of info about many, many horses, jockeys, owners, trainers, what have you. The horses had great names: my favorites so far are Winkipop and Humorist.