"My Republican vote [in the 1972 presidential election] produced little shock waves in the New York intellectual community. It didn't take long - a year or two - for the socialist writer Michael Harrington to come up with the term "neoconservative" to describe a renegade liberal like myself. To the chagrin of some of my friends, I decided to accept that term; there was no point calling myself a liberal when no one else did." [Irving Kristol, "Forty Good Years," "The Public Interest," Spring 2005]
The term is attested from 1960, but it originally often was applied to Russell Kirk and his followers, who would be philosophically opposed to the modern neocons.