Graffiti
Scribble, Sketch
noun, verbal noun
When writing just happens to "fall" on the wall (whoops, wasn't me who did that, how strannnnge) it's writing that's not supposed to be where it is. This kind of writing is graffiti.
This is a jukugo word, which usually means on'yomi readings from the kanji. 書, however, is kun'yomi in this case and also has rendaku. So just remember that it's read as が for this vocab and you'll be fine.
ここに落書きをしたのは誰だ？
Who did the graffiti here?
いやいや、ただの落書きですよ。
No, no, no, it's just a doodle.
新宿駅の地下道には、昔、たくさんの落書きがしてありましたが、今はほとんど見当たりません。
There used to be lot of graffiti along the underground passages at Shinjuku Station a long time ago, but we rarely see them now.