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..."
You: "STFUW!"