mid-13c., from Old French perle (13c.) and directly from Medieval Latin perla (mid-13c.), of unknown origin. Perhaps from Vulgar Latin *pernula, diminutive of Latin perna, which in Sicily meant "pearl," earlier "sea-mussel," literally "ham, haunch, gammon," so called for the shape of the mollusk shells.
Other theories connect it with the root of pear, also somehow based on shape, or Latin pilula "globule," with dissimilation. The usual Latin word for "pearl" was margarita (see margarite).
For pearls before swine, see swine. Pearl Harbor translates Hawaiian Wai Momi, literally "pearl waters," so named for the pearl oysters found there; transferred sense of "effective sudden attack" is attested from 1942 (in reference to Dec. 7, 1941).