What is the X in X-mas?
c.1400, "burning, corrosive," from Latin causticus "burning, caustic," from Greek kaustikos "capable of burning; corrosive," from kaustos "combustible; burnt," verbal adjective from kaiein, the Greek word for "to burn" (transitive and intransitive) in all periods, of uncertain origin with no certain cognates outside Greek. Figurative sense of "sarcastic" is attested from 1771. As a noun, early 15c., from the adjective.
caustic caus·tic (kô'stĭk)
A hydroxide of a light metal.
A caustic material or substance.
Capable of burning, corroding, dissolving, or eating away by chemical action.
Of or relating to light emitted from a point source and reflected or refracted from a curved surface.
Causing a burning or stinging sensation.