Quinion doesn't say that Plutarch is wrong about the meaning of sycophant in his day (i.e., informer) or that it comes from the expression fig-shower. Just that he's wrong about why fig-shower should mean informer.

The LSJ on Perseus has this comment in the entry for sukophantes:

From sukon phainein, orig. used of denouncers of the attempted export of figs from Athens, acc. to Ister 35, Plu. Sol.24, 2.523b; orig. of citizens entrusted with the collection of figs as part of the public revenues of Athens and the denouncing of tax-evaders, acc. to Philomnest.1; of denouncers of figs which had been stolen from the sacred fig-trees during a famine and had become cheap, the famine having passed, Sch.Ar.Pl.31, cf. Fest. p.393 L.; these and modern explanations are mere guesses; the word first in Ar. but implied by sukopedilos.)

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