The Collaborative International Dictionary of English v.0.48:

Crackajack \Crack"a*jack`\, n. [Now usually spelled
   crackerjack.] [prob. from crack + jack, a man.]
   1. An individual of marked ability or excellence, esp. in
      some sport; as, he is a crackajack at tennis. [Slang]
      [Webster 1913 Suppl.]

   2. A preparation of popped corn, candied and pressed into
      small cakes. [U. S.]
      [Webster 1913 Suppl. +PJC]

The Collaborative International Dictionary of English v.0.48:

Crackajack \Crack"a*jack`\, a. [Now usually spelled
   crackerjack.]
   Of marked ability or excellence. [Slang]
   [Webster 1913 Suppl. +PJC] crackbrained

The Collaborative International Dictionary of English v.0.48:

Crackerjack \Crack"er*jack`\, a. [also spelled crackajack.]
   Of marked ability or excellence. [Slang]
   [Webster 1913 Suppl. +PJC]