Other forms: slathered; slathers; slathering
When you slather something, you apply or spread a lot of it. So you might slather your hamburger with ketchup, or slather shampoo on your damp head.
Before it came to mean "spread liberally" in the nineteenth century, people in parts of England used slather to mean "slip or slide." Today we use this informal verb to describe smearing or rubbing, so we slather paint on a fence to cover up graffiti, slather aloe on our sunburnt skin, and slather peanut butter and jelly on bread.