The Collaborative International Dictionary of English v.0.48:

Dern \Dern\, n. [Etymol. uncertain.]
   A gatepost or doorpost. [Local Eng.] --C. Kingsley.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dern \Dern\, a. [See Dearn, a.]
   1. Hidden; concealed; secret. [Obs.] "Ye must be full dern."
      --Chaucer.
      [1913 Webster]

   2. Solitary; sad. [Obs.] --Dr. H. More.
      [1913 Webster]