Some English Words or Idioms about "Jack".
I find that Jack is very busy because I see him for several times recently.

1. Jack is as good as his master.【夥計不比老闆差】
2. All work and no play makes Jack a dull boy.【只工作不玩，聰明的孩子也會變傻】
3. Ball the jack.【孤注一擲】
4. Jack of all trades and master of none.【萬事皆通而一無所長的人/三腳貓】
5. Jack on both sides.【模棱兩可/兩面派】
6. Jackpot.【頭等獎】

Why is Jack, I'm curious.

Question

Some English Words or Idioms about "Jack".
I find that Jack is very busy because I see him for several times recently.

1. Jack is as good as his master.【夥計不比老闆差】
2. All work and no play makes Jack a dull boy.【只工作不玩，聰明的孩子也會變傻】
3. Ball the jack.【孤注一擲】
4. Jack of all trades and master of none.【萬事皆通而一無所長的人/三腳貓】
5. Jack on both sides.【模棱兩可/兩面派】
6. Jackpot.【頭等獎】

Why is Jack, I'm curious.

Answer

it is because in 17th century England Jack was a generic term to refer to"man". These idioms and proverbs are very old.

Also "I find that Jack is very busy because I see him for several times recently"  is not correct.  you mean "because I see him often"  "him for several times" is not correct

Answer

Thank you, Peachey and imk. Could you please give me some examples about the items 2,4 and 6, I wonder how to use them naturally.

Answer

2,4, and 6 are common. 1,3, and 5 I have never seen before.

Answer

Then there's also the towel-jack, and the jack for a car... and even toilets can be called "the jake".