The Collaborative International Dictionary of English v.0.48:

Crackerjack \Crack"er*jack`\, n. [also 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]

   Syn: jimdandy, jimhickey.
        [Webster 1913 Suppl. +PJC]

   2. A preparation of popped corn, candied and pressed into
      small cakes. The name Cracker Jack(TM) is now a trademark
      for a similar confection. [U. S.]
      [Webster 1913 Suppl. +PJC]

   3. something excellent of its kind.

   Syn: jimdandy, jimhickey.
        [WordNet 1.5]

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]

WordNet (r) 3.0 (2006):

crackerjack
    n 1: someone excellent of their kind; "he's a jimdandy of a
         soldier" [syn: jimdandy, jimhickey, crackerjack]
    2: something excellent of its kind; "the bike was a jimdandy"
       [syn: jimdandy, jimhickey, crackerjack]