The first usage of douchebag/douche bag that I could find in the pejorative sense dates back to at least 1951, in the classic novel by James Jones, (1) From Here to Eternity (here it is used an adjective):
“The trouble with you, Pete,” the voice that did not seem to come with him but from the cigarette, said savagely, “is that you can’t see further than that douchebag nose of yours.”
—From here to Eternity, by James Jones, 2012 (1951) Dial Press, p. 307. NOTE: The novel is set in 1941.
So douchebag seems to have been used in a vulgar context as far back as World War II or thereabouts. It’s worth noting, however, that this is the ONLY usage of the type found in 1950’s literature: all other examples of douchebag/douche bag refer to medicine or hygiene. I doubt the term was in popular currency at the time.
The next such usage doesn’t appear until 1964, in a stream-of-consciousness passage of another famous novel, Hubert Selby’s (2) Last Exit to Brooklyn:
“…and she yelled to Jack to come on and she’s not fuckin’ blind not like that fuckin’ douchebag he was with and someone yelled we’re coming and she was dragged down the steps …”
This dialogue occurs about mid-book. By the author’s request there are no page numbers to this book.
Sources:
1) Google Books
2) Google Books’%20douchebag%20he%20was%20with%20and%20someone%20yelled%20we’re%20coming%20and%20she%20was%20dragged%20down%20the%20steps%22&f=false