(also slieveen, sleiveen) an untrustworthy or cunning person, from the Irish slíbhín. Used in Ireland and Newfoundland. (OED)
A sly person. The term is often used in politics or business to refer to someone who uses smooth talk to get their own way, or borderline-nefarious means for personal benefit. It comes from the Irish word “slíbhín”, which means a trickster, particularly a silver-tongued one.