Definition from Wiktionary, the free dictionary
Alternative forms 
- Exceptionally fine or excellent; top-notch; high quality.
- Expert, top-rated or high-performing.
- (US, slang) Referring to clothing items (pants, jersey, collar) that comprise the traditional blue uniform of the enlisted men of the US Navy, following the sailor uniform worn by Sailor Jack on boxes of Cracker Jack snacks.
crackerjack (plural crackerjacks)
- An exceptionally fine or excellent thing or person.
- An expert or top-rated individual (e.g., a marksman)
- (US, slang, in the plural) The traditional blue uniforms of the enlisted men of the US Navy.
- (US, slang) A sailor.