Sardonicism is the expression of derision and emotionlessness through a sinister and ferocious laugh. In some uses, the laugh is intentional, meant to inspire terror in those who see it; in others, it is an involuntary or convulsive side effect of a particularly heartless action. Like sarcasm, sardonicism is derisive, but sardonicism implies a cruelty and maliciousness that sarcasm, with its edge of humor, does not. Sardonicism is often seen in its adjectival form in the phrase "sardonic laugh," a translation of the Latin "Risus sardonicus", a convulsive laughter.
The word first appears in Homer in Ancient Greek, as "sardánios," "the scornful laughter of an angry person, with own pain or own damage"). Odysseus, on his return to Ithaca, laughs sardonically to himself when attacked by his wife's erstwhile suitors. The roots of this word are uncertain, though it may be related to σαίρω (sairō) "I grin". However, by the time the word had appeared in Latin, its form had changed to "sardonius," through the influence of a Sardinian plant called "sardonion" in Greek, literally, "plant from Sardinia." This plant was believed to affect those who consumed it with unstoppable convulsive laughter and, ultimately, death[1]. From Latin, the phrase passed into French (French: ris sardonien, sardonique) and English.
A popular folk etymology for the word is that the Sardinians (Latin: Sardoni), mythologized by the Greeks as fierce barbarians, would laugh while killing the elderly.