Source : Webster's Revised Unabridged Dictionary (1913)

Thick \Thick\ (th[i^]k), adv. [AS. [thorn]icce.]
   1. Frequently; fast; quick.

   2. Closely; as, a plat of ground thick sown.

   3. To a great depth, or to a greater depth than usual; as,
      land covered thick with manure.

   {Thick and threefold}, in quick succession, or in great
      numbers. [Obs.] --L'Estrange.