Confession time: it's not an original piece of vulgar artistry.
This past summer my family and my sister's family were at a lake. A couple of tools in their 20s were in the lake with some teenagers, all related. They were swearing and screaming stuff to one another- we had 5 girls that were between 4 and 10. We asked them to please stop, twice, and the second time one of the guys went off about how he served for this country and for his right to say what he wants when he wants because its the freedom he fought for.
So yeah, that was odd.
Anyways, a few minutes later we as a family were talking about the canoe we had just bought off craigslist and from 50yards away the tool yelled over 'yeah, I bet its a douchcanoe!'
We laughed, hard, at that one since it actually fit the topic.
The next week we christened the canoe the DC and had it logoed as such on the side.
That was the first time I had heard the term and somehow have seen it mentioned a few times since.