The Collaborative International Dictionary of English v.0.48:

Thick \Thick\ (th[i^]k), adv. [AS. [thorn]icce.]
   1. Frequently; fast; quick.
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   2. Closely; as, a plat of ground thick sown.
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   3. To a great depth, or to a greater depth than usual; as,
      land covered thick with manure.
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   Thick and threefold, in quick succession, or in great
      numbers. [Obs.] --L'Estrange.
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