From the Yiddish, taken to mean a promiscuous woman; a slut; a whore. Very similar to the Yiddish term 'koorvah', although generally understood to be somewhat less severe in nature, in that koorvahs do "it" for money, whereas nafkas more often seek physical or emotional gratification.
"Kiki, you nafka, you!" -Lenny Bruce
By David Ferris