1. Of snow or ice, to melt partially, becoming sloppy and wet in the process. The ice along the river began slushing up as the cold snap finally subsided.

2. To cause something to turn into slush. In this usage, a noun or pronoun can be used between "slush" and "up." I like to freeze fruit juice in ice cube trays, then put them in the blender and slush them up with some fresh fruit and yogurt. The warm sun began immediately slushing up the snow in the front yard.

3. To cover or splatter something with slush. In this usage, a noun or pronoun can be used between "slush" and "up." Would you go out and scrape the melted snow off the windscreen for me? I don't want to slush up my new suede boots. The workers slushed my floor up as they trudged in and out of the house in their work boots.