In its original sense the chief uses are such as the quick and the dead, of the Apostles' Creed, a quickset hedge, i.e. consisting of slips of living privet, thorn (NB. In Northern Europe, quick-thorn refers to the tree hawthorn (Crataegus monogyna), that is commonly used for hedging purposes) etc., the quick, i.e. the tender parts of the flesh under hard skin or particularly under the nail. The phrase quick with child is a conversion of with a quick. i.e. living child.
From the sense of having full vigour, living or lively qualities or movements, the word got its chief current meaning of possessing rapidity or speed of movement, mental or physical. It is thus used in the names of things which are in a constant or easily aroused condition of movement, e.g. quicksand, loose water-logged sand, readily yielding to weight or pressure, and quicksilver, the common name of the metal mercury.