an advance of money; loan.
By law the whole of the seafaring population of Britain was liable to serve the King at sea, and if a man was given an “imprest” or advance payment by a King’s agent — “taking the King’s shilling” — he had to serve. — Dudley Pope, Decision at Trafalgar, 1999
Off you go, see the foreign editor, get an imprest—don’t imagine you can squander money on this trip, though! — Eileen Arnot Robertson, Justice of the Heart, 1958
Imprest entered English in the late 1500s from the Anglo-Norman and Middle French prest, which described the action of lending.