AFAIK, the term was coined by The Fortean Bureau's Jeremy Tolbert, with not insignificant contributions by FB staff members tanaise and buymeaclue. (And I'm probably forgetting somebody.)
There's a philosophy behind it that I find both charming and useful in its practicality; basically, the idea is to exploit genre conventions (from genres as necessary) rather than allowing one's self to be limited by them.
I think this idea reflects a sensibility that's common among younger writers, and to my mind, a bit reminiscent of the New Wave in Science Fiction--cheerful rapine and exploitation of everything artistic that comes within reach. And it's not just literature that is subject to piracy on the high seas. cpolk talks a lot about appropriation of techniques from painting and cinema, for example. I like white space; she likes line of direction.
I think the genre pirate sensibility is a useful antidote to the mythologizing of writing. We accept that a painter needs to practice, to copy the masters, to develop his own style. There seems to be an idea, though, that writing is all about 'talent,' that it's something one is either good at, or isn't.
I actually think there's some merit in copying the masters, as it were. Not only does it lead to an awareness of which aspects of style are technique, and which voice, but (dirty sekrit) for a new writer, it seems to be easier to sell pastiche--in my theory, because the editors know how to read a good pastiche, whereas they need to learn to read a writer's particular style. (/dirty sekrit)
The writer can use the pastiche to earn Author Points, in other words--where Author Points = the trust the reader has in the writer (which trust has to be earned). (Author points then get spent when the writer wants to do something weird or unusual and needs the reader to hang on through it.)
I'm also reasonably convinced that the Genre Pirate thing relates to Steve Brust's Cool Shit Theory of Literature. Somehow. And also the quote New Pulp unquote.
But I think I'd need visual aids to describe what I mean.
And in other news, happy MLK day.