Left: body part 月/肉, right: night 夜 (On the roof 亠 is a person 亻/人 watching the moon 月 during the night.) The part of the body where (it is as) dark as night is the armpit.
Left: hand 扌, right: night 夜 (On the roof 亠 is a person 亻/人 watching the moon 月 during the night.) If the hand is in the 'night' it was tucked (into a pocket).
Radicals are shown here that are similar either in appearance or meaning.
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