stretch mouth, grimace, grin
咧 centers on stretching the mouth wide, producing a grimace or a grin. The core idea is a forced, exaggerated mouth movement.
咧 combines 口 (mouth) with 列, which likely contributes the sound and may suggest a stretching or arranging action. The exact historical development is uncertain.
The mouth 口 on the left is stretched wide by the right side 列, as if the mouth is being pulled into a grimace or grin.
For レツ, imagine stretching your mouth into a grimace while saying "let's" (レツ) — the forced mouth shape matches the meaning.
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