- 11a marine fish that is typically laterally compressed with a row of large spiky scales along each side. Jacks are important in many places as food or game fish. Also called pompano, scad.[originally a West Indian term]
- Family Carangidae (the jack family): many genera and numerous species. The jack family also includes the horse mackerel, pilotfish, kingfishes, and trevallies
- 14short for jackrabbit.
- 15US informalshort for jack shit.
late Middle English: from Jack, nickname for the given name John. The term was used originally to denote an ordinary man (jack1 (sense 6)), also a youth (mid 16th century), hence the 'knave' in cards and 'male animal'. The word also denoted various devices saving human labor, as though one had a helper (jack1 (sense 1), jack1 (sense 3), jack1 (sense 9), and jack1 (sense 10), and in compounds such as jackhammer and jackknife); the general sense 'laborer' arose in the early 18th century and survives in cheapjack, lumberjack, steeplejack, etc. Since the mid 16th century a notion of 'smallness' has arisen, hence jack1 (sense 4), jack1 (sense 5), jack1 (sense 7), and jack1 (sense 13)