stone slab used for washing clothes, an anvil
碪 centers on a heavy, flat stone surface used for pounding or shaping: a stone slab for washing clothes or an anvil for metalwork.
碪 combines 石 (stone) with 甚, which likely contributes the sound. The character refers to a stone tool used for pounding or as a base for hammering.
The left side is a stone 石, and the right side 甚 looks like a heavy block. Picture a large stone slab used for pounding clothes or as a sturdy anvil.
For チン, imagine a blacksmith striking an anvil with a sharp 'chin' sound: chin -> チン, as the hammer hits the stone anvil.