(pl.) "secret rites in the worship of certain Gk. and Roman gods," especially Dionysus, from M.Fr. orgies
(c.1500), from L. orgia,
from Gk. orgia
(pl.) "secret rites," from PIE base *werg-
"to work" (see urge
(v.)). The singular, orgy,
was first used in Eng. 1665 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.