Top: inside 内, person 人 - Or: twice 人[=rips], around 冂 which the meat is. (As a radical simplified to: 月, which looks equal as the radical for month/moon)
Mnemonic
The inside of a person is flesh. (Or easier: Image of marbled meat.)
The radical niku is typically at the left side of a composition and drawn as: 月, which looks equal to: "moon, month 月" but has the meaning 'body part'.
The radical tsuki stands for moon or month when used at the right side of a character. However the radical niku肉 that stands for body or body-part is in many cases simplified to 月 and thus looks equal as "moon".
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