piaster (n.)

late Old English plaster "a medicinal solid compounded for external application," from medical Latin plastrum, shortened by loss of the original prefix from Latin emplastrum "a plaster" (in the medical as well as the building sense), from Greek emplastron "salve, plaster" (used by Galen instead of the more usual emplaston), noun use of neuter of emplastos "daubed on," from en- "on" + plastos "molded," verbal adjective from plassein "to mold" (see plasma).

The use in reference to the material composed of lime, water, and sand (with or without hair for binding), used for coating walls, is recorded in English from c. 1300, via Old French plastre, from the same source, and in early use the English word often had the French spelling. The meaning "gypsum" is from late 14c.; plaster of Paris "powdered calcinated (heat-dried) gypsum," which sets rapidly and expands when mixed with water(mid-15c.) originally was made from the extensive gypsum deposits of Montmartre in Paris. Plaster saint "person who makes a hypocritical show of virtue" is by 1890.

also shin-plaster, piece of paper soaked in vinegar, etc. and used by the poor as a home remedy to treat sores on the legs, the thing itself attested by 1771; from shin (n.) + plaster (n.). In U.S. history, it became a jocular phrase or slang term of abuse for devalued low-denomination paper currency (by 1817).