/ˈmæŋki/

From a concerned citizen, we learn a new bit of slang with the intensifier derived ultimately from the Latin mancus, which also lends us maimed, mutilated and mangled that signals something unpleasant, dirty or disgusting.
The bigger story of course behind this errant graffiti tag in a park in Edinburgh besides the etymology is that the swan is somehow taking it as a personal affront and wants to shield his cygnets (the article goes on to identify the father, a cob—the name of an adult male from the Middle English word cobbe for group leader, whereas an adult female is called a pen, as their plumage made the best quills and hence the writing implement) from such language. Across the Atlantic, the similar-sounding janky refers to poor quality or unreliable objects and has an unknown origin—perhaps from junky.