Definition of lunar caustic in US English:

lunar caustic

noun

Chemistry
archaic
  • Silver nitrate, especially fused in the form of a stick.

    • ‘At the time with which we are concerned lunar caustic was used as an astringent, in medicine and for cosmetic purposes, and for marking linen.’
    • ‘For a long time in the treatment of places to be cauterized, only the dreaded nitrate of lunar caustic was used to produce the caustic effect.’

Origin

Early 19th century: lunar in the sense ‘containing silver’.