This is an utterly obsolete medical word for a perfumed powder used in medicine to be sprinkled over the body to restrain sweating, reduce inflammation or to destroy body odour.
Though it continued to appear in dictionaries until the beginning of the twentieth century, it had by then gone out of use. But then, it was never common, appearing most often as brief references in medical dictionaries and encyclopedias. This is a rare occurrence in general writing:
But even though I might be able to assuage the pains of disease I could not remove the seeds of contagion which were evidently now lodged in the ship. I knew of no empasm that could disperse the fetid odours of death which were hourly rising from the hold below.
Remarkable Escapes of a Predestinated Rogue, in The Court Magazine, Dec. 1835.
It often appeared in medical literature alongside the closely related diapasm, also a scented powder made from aromatic herbs for sprinkling over the person. A diapasm was sometimes made into little balls that were strung together round the neck. A third word of similar form is cataplasm, an old term for a poultice.
All three are from the Greek plassein, to sprinkle or shape.