Of a surface, having low friction, often due to being covered in a non-viscous liquid, and therefore hard to grip, hard to stand on without falling, etc.
Oily substances render things slippery.
(by extension) Evasive; difficult to pin down.
a slippery person
Liable to slip; not standing firm.
Unstable; changeable; inconstant.
Wanton; unchaste; loose in morals.