youself: That may well be where the usage came from, but, according to Van Ronk (& in my limited experience) it is common in singer-songwriter circles, so called. (Van Ronk considers that term misleading, as do I, and suggests the substitute "new song", which seems to me impossibly vague. At any rate, it hasn't caught on.) In the footnote I cited, he says:

"Incidentally, the new song movement has so completely taken over the remains of the folk scene that I recently heard a friend say of someone who, like myself, is best known for interpreting material written by others, 'Oh, she only does "covers"!' I had a sudden vision...."