Slither means to move in a sideways motion, usually silently. Snakes, of all kinds, slither, from the original snake in the Garden of Eden, to that untrustworthy-looking individual who approaches you in the street.
Slither comes from the Old English term slidrian, meaning "to slide on a loose or gravely surface." While slide is in some ways similar to slither, it lacks its truly nasty moral and physical associations. It's a safe bet that slitherers do not have your best interests at heart.
