Quicksand is a colloid hydrogel consisting of fine granular material (such as sand or silt), clay, and water. Quicksand forms in saturated loose sand when the sand is suddenly agitated. When water in the sand cannot escape, it creates a liquefied soil that loses strength and cannot support weight.
I took it as you would disappear quick(ly) if you stepped in it as shown in the old films. It is in fact a fallacy but the word fitted (and Les came up with the same answer).