In Dutch we use the same word, 'douche' for shower. We took it directly from French.
But in Anthrax' case:
Slang use: Douchebag, or simply douche, is considered to be a pejorative term. The slang usage of the term dates back to the 1960s. The term refers to a person with a variety of negative qualities, specifically arrogance and engaging in obnoxious and/or irritating actions without malicious intent.
Oh, and there was of course this in '85 S.O.D.
Why do you play so fast...to be cool?
If you think you'll last...you're just fools
You try to be something...that you're not
And all you do is fill our ears...with rot
You claim to play hardcore
Well that's a bunch of shit
You're just a bunch of snot nosed kids
A real group of tits
You make your bed you lay in
I bet it's made of shit
You think you're all so macho
I bet you all have clits
[CHORUS]
You're just a douche crew
You're just a douche crew
You're just a douche crew
You're just a douche crew
We offered you our hands
Yu just turned your backs
You're just a bunch of poser douchbag
Faggot, dickless, hacks
Wearing all their douchy clothes
They like to watch each other pose
They're all the same, so fuckin' lame
We'll shove their spikes right up their holes