Some common expressions with ‘jack’:
The phrase ‘Union Jack’ refers to the flag of the United Kingdom.

The expression ‘Jack-o’-lantern’, which we now use for a lantern made from a hallowed pumpkin displayed on Halloween, originally meant ‘jack with a lantern’ used for a night watchman.

The expression ‘Jack-in-the-box’ is a children’s toy — a box with a crank and when the crank is turned, it sings a melodious song and at the end of the song, the box opens suddenly and a clown-shaped figure pops up which surprises the onlookers. (There is a fast food restaurant chain with the same name.)

The ‘Jack of diamond’ is a card in a pack/suit of playing cards —  with a picture of a servant or soldier, below the value of the Queen.

The expression ‘every man Jack’ means all the members of a group without exception.

‘Jack O’ the clock’ is a small figure that comes out of a mechanical wall-clock in which a figure or figures come out to strike a bell every hour.

And the saying: ‘Jack of all trades, master of none‘ is used to refer to a workman with some skill in several fields but not very good at in any particular field; for example, a workman who knows plumbing, electric work, carpentry, mending locks, etc. but he does not have thorough knowledge or expertise in any one these fields.

A ‘jumping jack’ is a kind of toy, a small figure of man or other animal whose body parts are joined loosely and can be made to move up and down by pulling strings attached to it. And ‘jumping jack’ is also used for a kind of physical exercise in which a person moves his/her arms and legs while jumping up and down.

Audio Jack and jacks are also used to refer to  connectors in an electrical device.

Note: ‘Jack’, always with a capital ‘J’, is a name of a boy or man, originally and in the present usage a pet name for John (also ‘Jackie’), which has become an independent proper name, e.g.  ‘Hey, Jack, what’s up.’, ‘Jack Martin’, ‘Jackie Chan’, etc. For an article that gives us some statistics and origin of this word as a name of a man or boy, please click here.  ‘Jack’ is also used to refer to a card in deck of playing cards below the rank of ‘queen’.