orgy (n.)

1560s, orgies (plural) "secret rites or ceremonies 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. This is traditionally considered a derivative of the root of ergon "work, activity," from PIE root *werg- "to do," but perhaps it should be connected to the root of orgy, or it might be a foreign word. The singular was rare in Latin and Greek; in English, orgy was first used 1660s, in the extended sense of "any licentious revelry, a wild carousal." OED says of the ancient rites that they were "celebrated with extravagant dancing, singing, drinking, etc.," which gives "etc." quite a workout.