Sarcastic and sardonic are similar, but not precisely the same, in meaning.
- Sarcastic: “marked by or given to using irony in order to mock or convey contempt.”
- Sardonic: “characterized by bitter or scornful derision; mocking; cynical; sneering: a sardonic grin.”
The core factor in sarcasm is verbal irony—essentially saying the opposite of what you mean. Sardonic language overlaps, but by definition tends to be harsher and more scornful and may not necessarily be marked by irony.
Regardless, both of them have very interesting origins:
“Sarcastic” comes to English in the 1500s from the Late Latin sarcasmus, which in turn came from the Greek sarkasmos, meaning “a sneer, jest, taunt, mockery.” But even earlier was the Greek sarkazein, which literally meant “to strip off the flesh” (with the word sarx meaning “flesh”), suggesting cutting or biting humor that reveals the intention “beneath the skin,” or beneath the literal meaning of the words.
“Sardonic,” on the other hand, came to English a bit later via the French sardonique, which is from the Latin phrase Sardonius risus (preceded by sardonios gelos in Greek), meaning “Sardinian laughter.” The implication was bitter or scornful laughter because the Greeks believed that eating a plant they called sardonion (literally a plant from Sardinia) caused facial convulsions resembling those of sardonic laughter, usually followed by death.