to let drop
沰 centers on letting something fall or drop, often in a liquid context given the water radical.
沰 combines 氵 (water) with 石 (stone), likely suggesting a drop of water falling onto a stone. The exact historical development is uncertain.
Water 氵 beside a stone 石: picture a single drop of water letting go and falling onto the stone below.
For タク, imagine a drop of water falling onto a stone with a soft 'taku' sound: taku -> タク.