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.
a new word formed by joining two others and combining their meanings; "`smog' is a blend of `smoke' and `fog'"; "`motel' is a portmanteau word made by combining `motor' and `hotel'"; "`brunch' is a well-known portmanteau"