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]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "crackerjack":
   ace, adept, beaut, brick, corker, crack, daisy, dandy, darb, dilly,
   doll, dream, expert, first-rater, genius, good Joe, good egg,
   good guy, good hand, great, honey, humdinger, killer-diller,
   knockout, likely lad, lollapaloosa, lulu, magician, mahatma,
   man of genius, master, master hand, masterful, masterly,
   mastermind, nice guy, nifty, no slouch, past master, peach, pip,
   pippin, practiced hand, prodigy, pussycat, sage, skilled,
   skilled hand, skillful, star, stout fellow, superstar, sweetheart,
   the nuts, topnotcher, trump, virtuoso, whiz, wizard