"nacreous mass formed in the shell of a bivalve mollusk as a result of irritation caused by some foreign body," early 14c., perle (mid-13c. as a surname), from Old French perle (13c.) and directly from Medieval Latin perla (mid-13c.), which is 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).
Used from 14c. of anything valuable or of the finest kind; from mid-15c. of something small, round, and glistening white. 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).