An I.O.U.; a bill of acceptance. “Hard,” means hard cash. “Did you get it stiff or hard?” means by an I.O.U. or in cash. Of course “stiff” refers to the stiff interest exacted by money lenders.
“His `stiff' was floating about in too many directions, at too many
high figures.” —
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Stiff from Infoplease: