crack⋅er⋅jack / ˈkræk / Show Spelled Pronunciation [ krak -er-jak] Show IPA Informal . –noun 1. a person or thing that shows marked ability or excellence.
–adjective 2. of marked ability; exceptionally fine.
Origin: 1890–95, Americanism ; earlier
crackajack, rhyming compound based on
crack (adj.);
-a- as in
blackamoor; jack1 in sense “chap, fellow”
crack·a·jack Slang crackerjack .
crack·er·jack Slang crack first-rate + jack crack'er·jack' n.
The American Heritage® Dictionary of the English Language, Fourth Edition
Copyright © 2009 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Slang Dictionary
crackerjack
mod. Fred is a crackerjack stockbroker, but his personal life is a mess.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition.
Copyright 2007. Published by McGraw Hill.
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[krak-er-jak] 
Informal.