c. 1400, from Old French mosaicq "mosaic work," from Italian mosaico, from Medieval Latin musaicum "mosaic work, work of the Muses," noun use of neuter of musaicus "of the Muses," from Latin Musa (see muse). Medieval mosaics were often dedicated to the Muses. The word formed in Medieval Latin as though from Greek, but the (late) Greek word for "mosaic work" was mouseion (Klein says this sense was borrowed from Latin). Figurative use is from 1640s. As an adjective in English from 1580s. Related: Mosaicist.
"pertaining to Moses," 1660s (earlier Mosaical, 1560s), from Modern Latin Mosaicus, from Moses.