Picture a hand moving fast and loose across paper. Whatever comes out of that hand — letters, loops, lines — comes out rough, because speed and care are pulling in opposite directions. Scribble names that fast loose movement first. Then it also names what the movement leaves behind on the page. Notice what the word does not say: nothing about the writing being wrong, and nothing about the person. A scribbled note can be perfectly good English, and a child's scribble is exactly what a small hand is meant to make.
From the word’s history
Wiktionary takes scribble back through Middle English scriblen to Latin scrībō, to write, and analyses the modern word as scribe plus the frequentative ending -le, an ending that adds the idea of doing something over and over. Write-write-write, quickly and repeatedly, is exactly the hand movement the word still describes.
Wiktionary 把 scribble 追到中古英語 scriblen，再回到拉丁文 scrībō（寫），並把它拆成 scribe（書寫）加上 -le 這個表示「反覆、連續做」的字尾。「一直寫、一直寫」的味道，正好接上今天「手動得很快、筆畫就散開」的畫面。
Wiktionary, English 'scribble' etymology 1 (packet etymology_text; source_ref kaikki:3f6f69a190036ae4:662678199:13365)
登入就可以看每一支怎麼連回核心畫面。