Sardonicism (connected with guffaw) characterizes —as distinct from Sarcasm— not a contumely or bitter, but a ferocious, painful derision.
The aborigines of Sardinia Sardoni would kill their elderly and laugh while doing this. This is called sardonic laughter, or the infamous "Risus sardonicus", a convulsive laughing, with which the soul is noninvolved. riso sardonico, ris sardonien, sardonique.
It was already established in the Ancient Greece. They also differentiated between cynic (kynikós "to the world opinion of the cynics"), sardonic (sardánios "the scornful laughter of an angry person, with own pain or own damage") and sarcastic (sarkastikós "derision, hurtful speech"). So Odysseus —after he had at last returned— laughed sardonic to himself when he dodged a foot of a cow, which was thrown by a punter of his wife.
Occasionally, the convulsive laugh is traced back to a bitter Sardinian plant called "sardánion". The Meyers Encyclopedia from 1888 says Sardoniasm was a:
The term sardonic (sardanios) "bitter or scornful laughter", which is often cited as deriving from the Sardinian plant (Ranunculus sardous), known as either σαρδάνη (sardanē) or σαρδόνιον (sardonion). When eaten, it would cause the eater's face to contort in a look resembling scorn (generally followed by death). It might also be related to σαίρω (sairō) "I grin".