Caustic means harshly critical or sarcastic in a way that burns or stings, like acid eating through metal. When someone makes a caustic remark, their words are designed to wound. A teacher with a caustic sense of humor might say, “I'm so glad you decided to join us today” to a student who arrives late, in a tone that makes everyone wince.

The word comes from chemistry, where caustic substances like lye can burn skin and dissolve materials. This scientific meaning gives us a perfect picture of what caustic words do: they corrode relationships and damage feelings. A caustic comment is deliberately hurtful, going beyond mere honesty or bluntness to inflict pain.

You might encounter caustic humor in some books or movies, where characters trade cutting insults. The related noun is causticity, which describes this quality of burning harshness.