Well, "slithy" means "lithe and slimy." "Lithe" is the same as "active." You see it's like a portmanteau - there are two meanings packed up into one word.
Lewis CarrollTo the Looking-Glass world it was Alice that said 'I've a sceptre in hand, I've a crown on my head. Let the Looking-Glass creatures, whatever they be, Come and dine with the Red Queen, the White Queen, and me.
Lewis CarrollThe master was an old Turtle--we used to call him Tortoise--' Why did you call him Tortoise, if he wasn't one?' Alice asked. We called him Tortoise because he taught us,' said the Mock Turtle angrily; 'really you are very dull!' You ought to be ashamed of yourself for asking such a simple question,' added the Gryphon; and then they both sat silent and looked at poor Alice, who felt ready to sink into the earth.
Lewis Carroll