means Slippery-smooth to the touch or the eye — and, of a person, suspiciously polished and persuasive in a way that's clever but not quite trustworthy.

from From Old English 'slician,' to make smooth or sleek, and tied to the same northern Germanic family that gave us 'sleek' itself — words for things glossy, oiled, and frictionless. For centuries it simply meant smooth and glistening; the sense of a person being slick — too smooth, all surface charm and no grip — is a later figurative slide, the same way we still call a smooth-talker 'slippery.'