A slicker is a waterproof garment you wear when it's pouring outside — in other words, a raincoat. Those dark clouds look ominous. Better grab your slicker before you go!
Any kind of raincoat can be called a slicker, but it's an especially appropriate term for a waxy, smooth garment that repels drops of rain, like the yellow slickers fishermen often wear. If someone is described as a city slicker, it means that they're sophisticated, accustomed to life in the city, and that they have fancy taste — so don’t ask them to help you milk the cows on your farm.