fork off

[jezmatron]

why, if I am to spend a considerable sum of money, must I 'fork out'?

[mandela]

it maybe because where theres muck theres money !

[Quizmonster]

Fork out, fork over and fork up have all been used for almost 200 years to mean the passing of money between people. The reference is clearly to gardening or farming, where earth, hay etc has to be moved from place to place in order to achieve some end.

[Bellringer]

It may possibly have some thing to do with "Mortons Fork"
Bishop Morton (or he may have been an Arch Bishop) was given the task of raising money for the King. I have forgotten which one. When he paid a visit to a nobleman if he was greeted with a sumptious spread and treated with luxury he reckoned that the nobleman was rich & could give a sizable donation to the King. If he was treated poorly with just the bare means he reckoned the nobleman must have a bit stashed away & was saving it ,so he could dig into his stash for the King.
He got them both ways hence the "Fork" two prongs.

[Quizmonster]

The earliest recorded use of the phrase, 'fork out', was in the 1830s and Bishop Morton was Lord Chancellor in the 1480s. I should think a 350 year gap is rather a long one to support Bellringer's suggestion...but who knows?
Much more likely, surely, is the relatively simple idea that you have to 'dig' into your pocket with your fingers - vaguely fork-shaped - and pull out money to be handed over in payment.