Definition of grey powder in US English:

grey powder

(US gray powder)

noun

  • 1Chalk mixed with finely divided mercury, originally used medicinally (especially as a laxative) and later used in fingerprinting (now historical).

  • 2Any of various other, usually aluminium-based, powders used in fingerprinting.

Origin

Mid 19th century; earliest use found in Cecilia Ridley (1819–1845).