To sweat a client. To make him bleed; to fleece him.
To sweat coin. To subtract part of the silver or gold by friction, but not to such an amount as to render the coin useless as a legal tender. The French use suer in the same sense, as “Suer son argent,” to sweat his money by usury. “Vous faites suer le bonhomme- tel est votre dire quand vous le pillez.” (Harangue du Capitaine la Carbonnade.) (1615.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894