1. To twist and undulate the body to propel the body across the top of something. The snake slithered over the hot sand, searching for bit of shade. We'll have to slither over the length of the field our bellies in order to avoid the tripwire surrounding the fortress.
2. To walk toward someone or something in a sliding motion, especially in a manner that makes others feel uneasy or distrustful. I could see the salesman start slithering over to us as soon as we paused to look at the used car. The sleazebag slithered over to the woman and plying her with lewd come-ons.