A poisonous plant called Herba Sardonia gave the English language the word sardonic and gave the Italian island of Sardinia its name. The island in turn gave English the word sardine, the name for a small fish of the herring family.
The dictionary defines sardonic as bitter, cynical, scornful. That which is sardonic has come to mean disdainfully or sneeringly derisive. A sardonic smile is contemptuous. Sardonic laughter, as used by Homer, is bitter, mocking laughter.
Unfavorable connotations come naturally from the namesake herb, «which renders men insane, so that the sick person seems to laugh». The acrid plant was reputed to be so very bitter as to cause convulsions that distort the face of one who eats it into a grin. But actually the twisted face reflected the throes of death.