“Utter barristers wear a stuff or bombazine gown, and the puckered material between the shoulders of the gown is all that is now left of the purse into which, in early days, the successful litigant ... dropped his ... pecuniary tribute ... for services rendered” ( Notes and Queries, 11 March, 1893, p. 124). The fact is that the counsel was supposed to appear merely as a friend of the litigant. Even now he cannot recover his fees.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Barristers' Gowns from Infoplease: