Not emacs (it means kill-region there, which deletes the text between the cursor and the mark and saves it in a holding area, fairly close to what windows does when you hit ^x, except the holding area is fancier).

This readline behavior follows a Unix tty driver. I'm not sure off the top of my head which one introduced it. I don't think it was on xenix but am pretty sure some 4.x bsd had it.