The term used here is the negative of the word used in the third of four phrases, that is, the righteous.
It refers to people who enjoy wickedness, unrighteousness, and evil. The angel says here that those who relish evil should go ahead and continue to do evil—forever. Indeed, exactly because they reject the saving work of Jesus Christ, they invariably will end up doing evil eternally. It should further be understood that being an evildoer is the natural bent of all (fallen) mankind as they align with the Evil One.
11 Let the evildoer still do evil, and the filthy still be filthy, and the righteous still do right, and the holy still be holy.”