When liquid things slosh, they move with a splash. If you stumble as you're carrying a cup of coffee to the table, your coffee may slosh over the edge of the cup.
Only something that's wet can slosh — water might slosh around in the bottom of a leaky rowboat, and soapsuds can slosh in a bathtub when a happy kid gets in. You can slosh tea into a mug, or slosh through puddles in your rain boots. The 1800's meaning of slosh was "slush or sludge," and the Middle English definition was "muddy place," probably from a combination of slush and slop.