Sui generis is Latin and means of its own kind: basically, in a class of its own, or unique.
You say it soo-I jenneriss.
It has various meanings in philosophy and biology etc, but as far as language goes it describes a work which doesn't fit into any particular genre.
These are rare, almost impossible to get published, and must annoy librarians very much indeed.
The other problem with them, of course, is that if they're successful then they end up starting their own genre and then ten-to-one in half a dozen years they're eclipsed by their offspring.
Phrase To Consider Today: sui generis. You know you've got one of these when someone describes it as seminal and then you realise it's never really produced any offspring. Like James Joyce's Ulysses, perhaps.