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.)