Sentence:  Behind him was gang-turf graffiti, elaborately spray-painted, transforming the wall into a billboard. One with a very clear message. The graffiti was pristine. That it had not been <WSD>overtagged</WSD> was a bold proclamation that the wall stood within undisputed territory. 
Meaning of the word marked with <WSD> tags:  To write a graffiti tag over another.