From The Art and Popular Culture Encyclopedia
The etymology of sardonicism as both a word and concept is uncertain. The Byzantine Greek Suda traces its earliest roots to the notion of grinning (sairō) in the face of danger, or curling one's lips back at evil. One explanation for a later morph to its more familiar form and connection to laughter (supported by the Oxford English Dictionary) appears to stem from an ancient belief that ingesting the sardonion plant from Sardinia (Sardō) would result in convulsions resembling laughter and, ultimately, death.
From the sardónios evolved the sardonius, thence the sardonique, and ultimately the familiar English adjectival form, sardonic.
Hemlock water dropwort
In 2009 scientists at the University of Eastern Piedmont in Italy claimed to have identified hemlock water dropwort (Oenanthe crocata) as the plant responsible for producing the sardonic grin. This plant is the most-likely candidate for the "sardonic herb," which was a neurotoxic plant used for the ritual killing of elderly people in pre-Roman Sardinia. When these people were unable to support themselves, they were intoxicated with this herb and then dropped from a high rock or beaten to death.