sardonic

From French sardonique, from Latin sardonius, from Greek σαρδόνιος, an alternative form of σαρδάνιος meaning bitter or scornful laughter. That Greek word is often traced to a Sardinian plant, called σαρδάνη or σαρδόνιον, which was said to make the eater's face contort into a scornful grin — usually just before they died.

The word began as a description of a deadly plant's effect on the face, and narrowed over time into a word for the expression itself — a mocking, scornful smile or tone, detached from the plant that first inspired it.

Some sources connect the root to Greek σαίρω (saírō), meaning 'to grin,' suggesting the word may have as much to do with baring teeth as with bitterness.