Merriam-Webster Collegiate Dictionary - caustic

 I. adjective  Etymology: Latin ~us, from Greek kaustikos, from kaiein to burn  Date: 14th century  1. capable of destroying or eating away by chemical action ; corrosive  2. marked by incisive sarcasm  3. relating to or being the surface or curve of a ~  • ~ally adverb  • ~ity noun Synonyms:  ~, mordant, acrid, scathing mean stingingly incisive. ~ suggests a biting wit ~ comments. mordant suggests a wit that is used with deadly effectiveness mordant reviews of the play. acrid implies bitterness and often malevolence acrid invective. scathing implies indignant attacks delivered with fierce severity a scathing satire.  II. noun  Date: 15th century  1. a ~ agent: as  a. a substance that burns or destroys organic tissue by chemical action  b. sodium hydroxide  2. the envelope of rays emanating from a point and reflected or refracted by a curved surface