Charlotte Higgins mentions two etymologies that she wishes she'd known for her book It's All Greek to Me, and which she'd gotten from the recent Why Socrates Died by Robin Waterfield and A Woman Scorn'd by Michael Burden, but I have to dispel these notions:
To paraphrase Waterfield: one of the vagaries of the classical Athenian judicial system was that it gave people the opportunity to make money out of threatening to take others to court.
These blackmailers were called sycophants. The origin of the word is this. Since the beginning of the 6th century it had been illegal to export food, except olives, from Athenian territory. Sometimes, though, people would try to smuggle figs over the border. If someone denounced you as a fig-smuggler, he was a sykophantes – a "tale-teller about figs". Waterfield: "If it was part of his purpose to ingratiate himself with the authorities, he was close to being a sycophant in the modern sense of the word."
Others have tried to link this with the fig sign as though the verb in Greek could be read as "to give the fig sign to."
But it is entirely clear to me that the word is built on sukon as it referred not to a fig but to a fig-like growth on the skin (e.g., a wart, a tumor). The verbal root phant- then adds the notion of revealing someone's 'warts', a metaphor that we still use. A sycophant to an ancient Greek was probably easily understood as someone who made known another's shortcomings or sins, whether real or trumped up.
As to the other etymology, namely for sardonic, the usual ancient etymology is derived from the plant sardanios, with reference to the contorted faces of those who've ingested the poisonous herb. The notion that it is named for the feigned joy of the victims of ritual child-sacrifice in Carthage is unthinkable on so many levels.
Each of these smacks of folk-etymology, and more than that the sort of folk-etymology that lends credence to the old saying that a little knowledge is a dangerous thing.