Old English cwic meant alive. The family developed movement associations: Middle English could apply it to shifting ground, while other uses came to emphasize speed and short duration. Quicksand preserves the older living image.
Sources
- Etymonline: quick
The entry gives living cwic, Middle English shifting-ground usage and later action completed in little time.
- Etymonline: quicksand
The compound joins the living sense of quick with sand.