The term is rooted in Judith Butler's work, particular the book Gender Trouble (1990). She argues that queerness is not necessarily about sexuality. She is responding to a version of feminism which tends to regard women's experience and identity as somehow unified and having an essential core. Gender (like all other supposedly essentialist characteristics we use to define ourselves) should be seen as fluid, free-floating, meaningful in a particular context but not transcendental. To be queer, then, is not to have an essence, but rather to perform gender in a particular way, in a transgressive way. To be queer is to proliferate meanings and identities, rather than to imagine that something causes or legitimates a particular gender performance.
David Halperin has said,
'Queer is by definition whatever is at odds with the normal, the legitimate, the dominant. There is nothing in particular to which it necessarily refers. It is an identity without an essence.'