This is just my humble scribble — please don't laugh.
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history
The term comes from a Tang dynasty story: the calligrapher Lu Xiang's young son scrawled all over his manuscripts. Lu sighed that the boy's scribbles looked like 涂鸦 (crow smears).
usage
In modern Chinese, 涂鸦 covers both street graffiti art and childish doodling. It can also be used as a humble way to describe one's own artwork: 信笔涂鸦 (casual scribbling).