偷偷摸摸 Origin — Chinese Character Wiki

tōutōumōmō

sneaky

tōu to steal

Phonosemantic compound. 亻 represents the meaning and 俞 represents the sound.

亻 Meaning component

rén person

亻 is a component form of 人.

俞 Sound component

yú, shù surname

Change in sound

偷 and 俞 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar in Old Chinese.
偷 *l̥ˤo
俞 *lo

Regular Modern

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	Middle Chinese	Old Chinese	Gloss
Baxter-Sagart	thuw	l̥ˤo	steal

Phonosemantic compound. 扌 represents the meaning and 莫 represents the sound.

扌 Meaning component

shou, shǒu hand

扌 is a component form of 手.

莫 Sound component

mò, mo do not

	Old Chinese
Zhengzhang	maː
	maːɡ