stupid
駘 centers on dullness or slowness of mind: stupidity. The meaning is narrow and negative, describing a lack of intelligence.
駘 combines 馬 (horse) with 台, which likely contributes the sound. The meaning 'stupid' may derive from the idea of a slow or dull horse, but the exact historical development is uncertain.
A horse (馬) with a platform (台) on its back might be too slow or stubborn to move, like a stupid animal that won't budge.
For タイ, imagine a stupid horse wearing a tie: tie -> タイ, and the horse looks foolish with a tie around its neck.
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