The Collaborative International Dictionary of English v.0.48:

Nutshell \Nut"shell`\, n.
   1. The shell or hard external covering in which the kernel of
      a nut is inclosed.
      [1913 Webster]

   2. Hence, a thing of little compass, or of little value.
      [1913 Webster]

   3. (Zool.) A shell of the genus Nucula.
      [1913 Webster]

   in a nutshell in a summarized and very abbreviated form; --
      of statments, descriptions, reports, and other
      communications; as, to describe the convention in a
      nutshell.

   To be in a nutshell or To lie in a nutshell,, to be
      within a small compass; to admit of very brief or simple
      determination or statement. "The remedy lay in a
      nutshell." --Macaulay.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

in a nutshell
    adv 1: summed up briefly; "gave the facts in a nutshell"; "just
           tell me the story in a nutshell"; "explained the
           situation in a nutshell"