Calling someone a “douche” derives from the term “deus,” which is a Zoroastrian term for “god.” Because Zoroastrianism has two gods, the name “deuce” was given to the “two” in a deck of playing cards by the Romans, who actually named all of the cards (the Jack, for example, retained his name). Roman soldiers in the field loved to play cards to pass the time, but they had to stay quiet so as not to alert the enemy to their presence. So, when one of them had to go off and relieve themselves, they couldn’t just announce it, so they would raise one finger as they left to indicate they were just going to urinate, while they would raise two fingers if they were going to defecate (thus giving rise to the terminology for “going #1/#2”).
So, in essence, calling someone a douche is calling them a shit. And that’s completely not true.