stone roller used to level fields
碡 refers to a stone roller used for leveling fields, a specialized agricultural tool. Its meaning is narrow and concrete, centered on this specific object.
碡 combines 石 (stone) with 毒, which likely contributes the sound. The character was created to name a stone farming implement.
A stone (石) roller that looks like it could be poisonous (毒) to weeds as it levels the field. Imagine a heavy stone cylinder crushing everything flat.
For トク, picture a stone roller that is especially toxic to weeds: toxic -> トク. The stone roller's weight makes it deadly to unwanted plants.
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