E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Garnish (g hard).
Entrance-money, to be spent in drink, demanded by jail-birds of new-comers. In prison slang garnish means fetters, and garnish-money is money given for the honour of wearing fetters. The custom became obsolete with the reform of prisons. (French, garnissage, trimming, verb garnir, to decorate or adorn.) (See Fieldings and Smolletts novels.)