Deet is what the little brown people on "Super Mario Brothers" do. They walk left and right and go "Deet, deet, deet!". A little brown person on Super Mario Brothers is referred to as a "goomba". Goomba's are the most common creatures that deet. Deet can also be used in the form of "getting someone", you say "I'll deet ya!".
"I hate the goomba's on Super Mario Brothers! They'll deet ya!"