You call someone a douche bag. What does that actually mean?

I feel like it’s one of those catchall terms that can refer to any person you don’t like. “He cut me off in traffic?! Douchbag!”...”He has an RBF and drives a Lamborghini. Douchbag!”

That they're wet behind the ears.
In English when we say someone is “wet behind the ears” we mean that a person is inexperienced or naive, or perhaps even immature or green. The phrase is often used in a business context to indicate that someone is not adequately prepared for the task at hand. It can also mean that someone is easy to fool. The origin of this phrase comes from birth. This post was edited by CosmicWunderkind at February 2, 2020 7:44 AM MST

Someone I find repugnant in some way; like we might imagine the contents of a bag of used douche water to be. 

Too graphic? 

It would mean I'm a fool, so you will never hear me call someone such a thing.  

When I was a little boy, about 4, I saw my mother's douchebag. It was in a drawer in the bathroom. It appeared to be black rubber, but I have no idea how it worked.