late Middle English: from Jack, pet form of the given name John. The term was used originally to denote an ordinary man (jack), 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 (jack, jack, jack, jack, 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 jack, jack, jack, jack.