means To move with a smooth, sliding, sinuous motion along a surface, the way a snake or eel travels.

from From Middle English slidder, 'to slip or slide,' itself rooted in Old English slidrian, a frequentative of slidan — to slide. So 'slither' is essentially 'sliding, again and again,' the verb wriggling its own way into being. It runs in the same slippery word-family as 'slide' and 'sled,' and the spelling drifted toward the 'th' sound over time, leaving us with a word that hisses a little when you say it.