Vagina, because the inner lips of a spread vulva form a shape resembling a canoe, and... the rest is obvious. Also used as a generic, fun-sounding term of insult in any context where one might use the word "cunt", "pussy", "twat", etc.
John said he was going to come out last night and help us TP the neighbor's yard, but he chickened out and didn't show. He's such a douchecanoe.
1. Any vehicle filled to capacity with a group of major douchebags. Usually a luxury SUV. Most often owned or purchased by the parents of one of the douche mcgouches inside. The group of individuals who pour out of the douche canoe can be affectionately referred to as a "shitstorm of douchebags."
If that douche canoe tries to swoop us in the Taco Bell line, I will go fisticuffs with that whole shitstorm of douchebags inside!