Keep watch on.
It is just an idiom and has no history.
About 400 BC in England , a
The image is of you pointing an eye at something or someone. It means to watch carefully.
spying or looking someone all the time
An excellent expression. To keep an eye, or to keep an eye on things means to maintain a watchful eye. You monitor what is happening with an appropriate zeal to the occasion. Another idiom would be, to keep an eye peeled. It means you are watching and protecting something, or someone.
An example of an idiom in the book "The Best School Year Ever" is "the apple of someone's eye." This idiom is used to show that someone is cherished or loved deeply. In the book, the Henderson children are seen as the apple of Mrs. Cavanagh's eye, despite their mischievous behavior.
No, the idiom is "turned a blind eye" as in "she turned a blind eye to his suffering." You don't change an idiom around or you lose the meaning.
No.
One example of an idiom in Chapter 1 of "Sign of the Beaver" is the phrase "the apple of his eye," which means someone cherished or loved above all others.
"Watch your back" means "Look out for what might be behind you." "I'll watch your back" means "Don't worry about what might be behind you, I'll take care of that."
Keeping an eye on someone means you are watching them. You could be watching them for their own safety, or you may find them suspicious. Example: "I wanted to go shopping, but Mom has me busy keeping an eye on my 2-year-old brother instead." Example: "Even though it was Halloween, I decided to keep an eye on the shopper with the ski mask, just in case they were a real robber trying to blend in with the people in costume."