Definition of jack

Word Originlate Middle English: from Jack, familiar form of the given name John. The term was used originally to denote an ordinary man, also a youth (mid 16th cent.), hence the ‘knave’ in cards and ‘male animal’. The word also denoted various devices saving human labour, as though one had a helper (sense (1), and in compounds such as jackhammer and jackknife); the general sense ‘labourer’ arose in the early 18th cent. and survives in lumberjack, steeplejack, etc. Since the mid 16th cent. a notion of ‘smallness’ has arisen, hence senses (4) and (5).