A douche bag is a piece of equipment for douching: a bag for holding the water or fluid used in douching (the word can also be used as a verb).
Terms such as douche bag, douche nozzle, douche cake, bag of douche, giant douche, and just plain douche are also popular derogatory terms of insult or ridicule in North America.
douche cake? i'm gonna have to try and fit that into conversation.