sleiveen or sleeveen (SLEE-veen) - n., a dishonest person, a trickster.
Mostly Irish and Newfoundland usage, and not surprisingly comes from Irish/Gaelic -- specifically slíghbhín or, to use modern spelling, slíbhín, from sliabh, mountain, with the strong implication that the person is rural. I can't help wonder whether the producers of Doctor Who knew the word when the created the
slitheen, though that's pronounced with stress on the second syllable. Not being Newfie or Irish, I can't give a very good usage example.
---L.