Once upon a time, six or seven hundred years ago there was a French allegorical poem called the Roman de Fauvel. It's about a horse called Fauvel who leaves his stable, moves into the biggest room in his master's house, and installs a custom made hayrack.
The point of the allegory is that the second the horse appears to be in charge everybody suddenly wants to be his friend. People come from far and wide to groom Fauvel.
For some reason this story was immensely popular and got translated into English, with the name Fauvel unchanged. However, the English word for grooming a horse was currying. It's a verb that is, apparently, still used in equestrian circles*. Thus a phrase developed. Anybody who sycophantically went round trying to be nice to a lord was said to be currying fauvel.
However, once the old French allegory had been forgotten, the idea of currying fauvel started to look rather odd. What the hell was a fauvel? Nobody knew any more.
Confusion about the phrase lasted until 1560 when the Geneva Bible came up with this line:
He thoght by this meanes to courry fauour with the worlde
It makes sense. Nobody knew what a fauvel was, but everybody knew that a sycophant wants a favour. So every since then favours have been curried and patronage has been vindalooed.
And here is the original favour being curried.
*I have never moved in equestrian circles myself. I can't seem to maintain a stable relationship.
P.S. There's an article about my book, The Etymologicon, in today's Sun.