Slime is a substance that's slippery, damp, and gooey, like the slime that grows on the bottom on a swimming pool if it's not cleaned often enough.
People are generally disgusted by slime, so if your dinner guest comments that your famous spinach soufflé looks like slime, you'll probably feel insulted. Like your spinach soufflé, slime is usually greenish in color, though the clear stuff a snail leaves behind is slime too. If you slime something, you cover it in a slimy substance. Experts believe that slime comes from the Old English lim, "sticky substance," which shares a root with the Russian slimak, or "snail."