Taken literally, it would be most unpleasant and irritating to have a flea in the ear, and the original sense of the phrase carried, figuratively, an even greater unpleasantness.
To be sent away with a flea in the ear indicated that one had received a sharp and stinging reproof or rebuff, often wholly unexpected.
Modern usage has somewhat softened the force of the phrase. Now we use it to carry no greater meaning than that of warning. To drop a flea in one’s ear often means merely to caution one against some procedure.
The phrase is very old. In English it has been used for at least five hundred years, and was then a direct translation from the French of that period.
Modem Frenchmen say “mettre la puce a l’oreille,” to put a flea in the ear. Rabelais, in Pantagruel, wrote it, “la pulce en l’oreille.” Maybe the Greeks had a similar expression.