No, don't search for this word on Urban Dictionary. By flacking, we mean art, even though repeated movements are also involved in this. Loose paving stones and cracks in the surface of roads are a nightmare for many road workers and less mobile people. Filling these holes and cracks with mosaics, literally and figuratively, has been elevated to an art form. What you call street art! Spot flacking in your area yourself. You can come across it in the most surprising places.