to harness a horse, a board in front of a carriage for the driver to lean on
絥 centers on equipping a horse for control: harnessing it, or the board a driver leans on in a carriage. Both meanings relate to preparing or using a horse-drawn vehicle.
絥 combines 糸 (thread) with 伏, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with harnessing and carriage equipment.
The left side 糸 suggests cords or straps, and the right side 伏 suggests a person leaning or bending. Picture using strong threads to harness a horse, or the driver leaning on a board held by straps.
For フク, imagine a horse being harnessed with a quick, firm 'fook' sound as the straps are pulled tight: fook -> フク.
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