Short for "Shut the fuck up, Willis". Occasionally used as an alternate response to what you talkin bout willis?
, however rather than not understanding the comment, the speaker would prefer the "Willis" to keep their god damn mouth shut.
Can be used against people whose actual name is Willis for greater effect, and additional points.
Willis: "Don't you think you'd better put the lid on that blender full of gunk before you plug it in?"
You: "STFUW!" *as you plug in the blender*
Willis: "I got another speeding ticket on the weekend, but didn't have to pay the fine because I'm awesome..."