means A wet, loose mixture of sand and water that loses its firmness under weight, trapping anything that struggles against it.

from A plain English compound dating back centuries, joining 'quick' and 'sand.' The trick is that 'quick' here doesn't mean fast — it carries its older sense of 'living' or 'alive,' the same one preserved in 'the quick and the dead' and in your 'quick' fingernail. So quicksand is literally 'living sand,' ground that seems to stir and breathe and swallow — a name that captures the old dread far better than the modern movie version deserves.