Normally, Jack is an old Anglo-French boy's name. However in England, jack became a general label meaning any common man. In this way it loses its capitalisation. This use of jack is similar to the use of the name Tommy for a British soldier or a Joe or GI Joe (general issue Joe) for an American soldier. The phrase every man jack means everyone who is an ordinary working class man. The common man in a deck of playing cards is called a jack.
Food associated with working class men can be called jack. Examples include jack or applejack, which is apple brandy and is cheaper and more lower class than French grape brandy and so suitable for the common man. Flapjacks are a workman's pancakes. Usually flapjacks are flat, round and made of wheat flour, milk and eggs; in some places they are made with oats and butter. Some dried fish and dried cheeses are also called jack.
Certain plants and animals use the jack- prefix. These include the jackass (male donkey or ass), jack-rabbit (US), jacks (UK male hares - females are jills), jackdaw (male crow – also some species of crow) and so on. Plants include jack-in-the-pulpit, jackpine and jackfruit (actually derived via Portugese jaca from Malaysian chakka, but altered to match the English jack). There are too many other less common examples to list here.
Tools normally used by common male servants are often called jacks, such as electrical connections, a soldier's jackboot, a powered hammer or hydraulic jack. Other examples include tools for lifting a car or removing work boots. A workman's pocket knife is a jackknife, instead of the smaller simple penknife carried by gentlemen. (A penknife needs to do no heavier work than sharpen a quill pen or a pencil.) A short flag pole is a jackstaff and flags flown from them are jacks. (The British United Kingdom flag is only properly called the Union Jack when so flown from a ship. Otherwise it is the Union Flag.) There are many historical uses of jack for certain jobs and tools no longer in use, or just used in some areas. There are far too many jack-words to list here.