1836, "old-fashioned," from French rococo (19c.), apparently a humorous alteration of rocaille "shellwork, pebble-work" from roche "rock," from Vulgar Latin *rocca "stone." Specifically of furniture or architecture of the time of Louis Quatorze and Louis Quinze, from 1841. If this is correct, the reference is to the excessive use of shell designs in this lavish style. For differentiation, see baroque. The general sense of "tastelessly florid or ornate" is from 1844.
Much of the painting, engraving, porcelain-work, etc., of the time has ... a real decorative charm, though not of a very high order in art. Hence rococo is used attributively in contempt to note anything feebly pretentious and tasteless in art or literature. [Century Dictionary, 1902]