낙서 – scribble

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The sign says, “scribbling forbidden.” 낙서 means scribble, doodle, or even graffiti (though more elaborate graffiti is simply called straight-up graffiti in Korean). The root word 낙 means fall, and the root word 서 means write – so, literally, 낙서 is ‘falling writing’. Note that 낙 shares the same root word as the 낙 in 낙엽 (falling leaves).
Anyway, 낙서 is a big deal among Korean elementary school students, and they often try to do it on each other’s books.
See also a separate K-signs post for the word 금지 (forbidden).