Late Middle English: from Jack, pet form of the given name John. The term was used originally to denote an ordinary man ( jack (sense 6)), also a youth (mid 16th century), hence the ‘knave’ in cards and ‘male animal’. The word also denoted various devices saving human labour, as though one had a helper ( jack (sense 1, jack sense 3, jack sense 9, jack sense 10), and in compounds such as jackhammer and jackknife); the general sense ‘labourer’ 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 (sense 4, jack sense 5, jack sense 7, jack sense 13).

• In the Middle Ages Jack, a pet form of John, was used to refer to any ordinary man, much as Tom, Dick, and Harry is today. By the 16th century it also meant a young man, and from this we get an alternative name for the knave (from the Old English for ‘boy’) in cards. In the 18th century a jack was a labourer, which gives us the second part of words like lumberjack (mid 19th century) and steeplejack (late 19th century). A jack was also an unskilled worker as contrasted with the master of a trade who had completed an apprenticeship, from which we get the saying jack of all trades and master of none. On the other hand, the apprentice could assert his equality with the words Jack is as good as his master. See also jockey

  A jack can also be a thing of smaller than normal size. Examples include the jack in bowls—a smaller bowl placed as a mark for the players to aim at—and jack as in Union Jack (late 17th century), which is strictly speaking a small version of the national flag flown on board ship. Jack-o-lantern as a name for a pumpkin lantern made at Halloween looks back to an earlier use of the phrase. In the 17th century it was a name for a will-o'-the-wisp (early 17th century), a light seen hovering at night over marshy ground, from another common first name—exchanging the idea of Jack with a lantern for Will with a ‘wisp’, or handful of lighted hay. I'm all right, Jack is an early 20th-century catchphrase used to express selfish complacency, which became the title of a film starring Peter Sellers in 1959. The Jack Russell terrier is named after a 19th-century English clergyman, known as ‘the Sporting Parson’, who was famed in hunting circles for breeding these terriers. Today a jackpot (late 19th century) is a large cash prize in a game or lottery. The term was originally used in a form of poker, where the pool or pot accumulated until a player could open the bidding with two jacks or better.

1990s: from hijack.

Late 19th century: from jack up 'give up'.