Sentence:  This person then ... acquainted him that it was the custom of the place for every prisoner, upon his first arrival there, to give something to the former prisoners to make them drink. This, he said, was what they called <WSD>garnish</WSD>; and concluded with advising his new customer to draw his purse upon the present occasion. 
Meaning of the word marked with <WSD> tags:  A fee; specifically, in English jails, formerly an unauthorized fee demanded from a newcomer by the older prisoners.