/krak"euhr jak'/ , Informal.
n.
1. a person or thing that shows marked ability or excellence.
adj.
2. of marked ability; exceptionally fine.
Also, crackajack .
[ 1890-95, Amer.; earlier crackajack, rhyming compound based on CRACK (adj.); -a- as in BLACKAMOOR; JACK 1 in sense "chap, fellow" ]