I’ve heard it more than a few times. What about it makes it insulting? How did it get its start as a derogatory term?
The first time I heard it was in the first Revenge of the Nerds movie back in the 1980’s.
Lots of insults don’t make much sense other than causing mental imagery. “Asshat” is one but I am sure there are many others.
Well, you could always legally change your name to that lovely word and see how it feels! :eek: (You do understand what the word means, right?) 
My WAG is that being compared to something that flushes out smelly vaginas is not a compliment. 
It’s a bag that holds the water used for vaginal irrigation. Is this something you would like to be called?
Rest assured it precedes that by decades.
Is that a trick question?
The SNL skit was first broadcast in May of 1980, though it was popular years before that.
I have always wondered what a douchebag really was, only having heard it as an insult in american movies. Now I know. Ignorance fought, I suppose.
Merriam Webster dates the slang usage from 1963.
Another source claims to quote the OED, which apparently gives 1967 as the year when the word came into more popular usage with a definition of An unattractive co-ed. By extension, any individual whom the speaker desires to deprecate.
A popular athlete at my high school was named Ducharme. He was, of course, universally known as “Douchebag”. This is quite possibly the only known positive use of the term.
Douche bag is closer to butt wipe than asshat.
It’s such a specific term for an insult, though. We don’t call people colostomy bags or catheters or tampons, so why douchbags? I join Love Rhombus in being puzzled by this word.
There was a book published in 1950, Goldin Dictionary of American Underworld Lingo. It cited douche-bag in the slang sense meaning “a term of utmost contempt for a woman.” It was based on research done from 1942-49.
The wife and I were participating in a pub quiz last night in The Pickled Liver, and one of the teams was named The Douchebags. (Our team was The Spawny Gits, and we won!)
The first time I heard the term was in 4th grade. A classmate screamed at another classmate who was teasing him, “Cut it out you douchebag”. The teacher had a fit and dragged the kid to the office. None of my classmates (nor I) had a clue.
ETA: Firefox spell checker says it is douche bag.
I have recently begun using the more colorful (imo) “Douche Nozzle” to describe someone of outstanding doucheitude.
Away from my wife, as she HATES the entire idea.
Douche bag is an insult probably for the same reason scum bag is. Both are items used in immediate proximity to the sexual organs, and then casually disposed of.
A scum bag is a condom, BTW.
That’s the first time I have heard that definition of scum bag.
Come to think of it, why is “cunt” an insult? I have nothing but the highest regard for the female bits, yet I don’t wanna be called a cunt.