drop, trickle, dripping, (kokuji)
雫 centers on a small amount of liquid falling or moving slowly: a drop, a trickle, or dripping. It is a kokuji created in Japan to express this specific image.
雫 is a kokuji (Japanese-made kanji) combining 雨 (rain) and 下 (down/below) to express a drop of liquid falling down. The historical development is straightforward as a logical compound.
Rain 雨 falling down 下 forms a single drop. Picture a raindrop sliding down a window: 雫 is that drop.
For ダ, imagine a drop of water falling with a soft 'da' sound as it hits the ground: da -> ダ, and the drop is 雫.