hypocaust (n.)

word-forming element meaning "under, beneath; less, less than" (in chemistry, indicating a lesser oxidation), from Greek hypo (prep. and adverb) "under, beneath; up from under; toward and under (i.e. into)," from PIE root *upo "under."

c. 1400, "capable of burning or destroying organic tissue, 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, which is of uncertain origin with no certain cognates outside Greek.

The figurative sense of "sarcastic, severely critical" is attested from 1771. As a noun "a caustic substance," early 15c., from the adjective.