c. 1400, "process of making patterns of inlaid work in hard materials," 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 often were dedicated to the Muses.
The word was formed in Medieval Latin as though from Greek, but the (late) Greek word for "mosaic work" was mouseion (and Klein says this sense in Greek was borrowed from Latin). Meaning "a piece of mosaic work" is from 1690s. Figurative meaning "anything resembling a mosaic work in composition" is by 1640s. As an adjective in English, "made of small pieces inlaid to form a pattern," from 1580s. Related: Mosaicist.