wheat
嘛 centers on wheat as a grain. The given meaning is a single concrete noun, so the core idea is simply the wheat plant or its grains.
嘛 is a phono-semantic compound: the mouth radical 口 suggests a semantic connection to speech or sound, while 麻 likely provides the sound マ. The character is used in Chinese for a sentence-final particle, but in Japanese it is assigned the meaning 'wheat'.
The mouth 口 beside 麻 (hemp) can remind you of a grain that is chewed: wheat is a grain you eat, and the mouth radical hints at consumption.
For マ, imagine a mouthful of wheat being ground into flour by a mill: the 'ma' sound of the mill grinding matches マ.
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