傻蛋 Origin — Dong Chinese Dictionary

shǎdàn

idiot

shǎ foolish

Depicts a person (亻) with a gap in their skull (𡕩), i.e. with no brains.

亻 Meaning component

rén person

亻 is a component form of 人.

mǎn gap between bones in skull

	Old Chinese
Zhengzhang	sqʰʷraːʔ
	sqʰʷraːs

Phonosemantic compound. 虫 represents the meaning and 延 represents the sound.

yán delay

Change in sound

蛋 and 延 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar in Old Chinese.
蛋 l'aːns
延 *la[n]

虫 Meaning component

chóng bug, insect

	Old Chinese
Zhengzhang	l'aːns