"apparently but not really proceeding from gaiety," especially of laughter, a grin, etc., 1630s, from French sardonique (16c.), from Latin sardonius (but as if from *sardonicus) as used in the phrase Sardonius risus, a loan-translation of Greek sardonios gelōs "of bitter or scornful laughter."
The Greek word was altered from Homeric sardanios (which is of uncertain origin) apparently by influence of Sardonios "Sardinian" (see Sardinia) because the Greeks believed that eating a certain plant they called sardonion ("plant from Sardinia") caused facial convulsions resembling those of sardonic laughter (usually followed by death). The etymology has been confused since ancient times and is much debated.
For nuances, see humor (n.). Earlier in same sense sardonian (1580s), from Latin sardonius. Of persons, "bitterly ironical, sarcastic," by 1833. Related: Sardonically.