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.
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