1560s, orgies (plural) "secret rites in the worship of certain Greek and Roman gods," especially Dionysus, from Middle French orgies (c.1500, from Latin orgia), and directly from Greek orgia (plural) "secret rites," especially those of Bacchus, from PIE root *werg- "to do" (see organ). The singular, orgy, was first used in English 1660s for the extended sense of "any licentious revelry." OED says of the ancient rites that they were "celebrated with extravagant dancing, singing, drinking, etc.," which gives "etc." quite a workout.