phylactery n.

late 14c., filaterie, philateria, "small leathern box or badge containing four Old Testament texts, worn by Jews on the arm or forehead as a reminder of the obligation to keep the Law," from Old French filatiere (12c.) and directly from Medieval Latin philaterium, from Late Latin phylacterium "reliquary," from Greek phylacterion "safeguard, amulet; a post for watchmen," noun use of neuter of adjective phylaktērios "serving as a protection," from phylaktēr "watcher, guard," from phylassein "to guard or ward off," from phylax (genitive phylakos) "guardian, watcher, protector," a word of unknown origin; Beekes writes that, based on the suffix -ax, "the word may well be pre-Greek." The custom of wearing it is based on a literal reading of scripture:

Therefore shall ye lay up these my words in your heart and in your soul, and bind them for a sign upon your hand, that they may be as frontlets between your eyes.  [Deuteronomy xi.18]