Joseph Campbell and copy-cat lexicographers say it's a mystery, but it's not if you apply a simple knowledge of Greek. Ctéis is the French transcription of Classical Greek kteis (genitive ktenos), which translates into English as comb, scallop, or female pubes. In Greek iconography the image of the scallop is rebus writing for the female pudendum, aka pubes, vulva, or pussy. Related is latin pecten, which has the same range of meanings. At Eleusis, which is Greek for "trysting" place, they worshipped the female naughty bit. That's why the Christians suppressed the cult of Demeter, not some much out of prudery but because it was "a false god". Still, if it feels good it must be a sin. Pass the oysters.