Aircraft have an instrument called a "Slip and Turn Indicator". The "slip" part in a turn shows a measure of how much the aircraft is slipping towards or away from the direction of turn (this should be zero in a properly balanced turn, as there should be enough rudder input to keep it in check). The slip indicator is a simple ball in a U-Shaped tube in the vertical plane, facing the pilot and with the base of the U being towards the feet. In the early days of aviation, and nowadays with student and newly trained pilots, one cause of crashing on the "turn-to-finals" is letting the aircraft enter a spin. This would not happen if the aircraft did not slip and thus the term "keeping your eye on the ball" refers to keeping the ball in the middle of the tube such that there is little or no slip and the chances of entering a spin are hugely reduced. "Keeping your eye on the ball" is commonly thought to have origins in football, although this is disputed and almost certain to be a falacy.
It is just an idiom and has no history.
The earliest recorded use in Modern English is in Sir Walter Scott's Old Mortality, 1816: "Poor Richard was to me as an eldest son, the apple of my eye."
To keep your focus on your main goal or objective.
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.
"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."
It is just an idiom and has no history.
About 400 BC in England , a
spying or looking someone all the time
The idiom "keep an eye out for" refers to watching for something or someone. An example of a sentence using the idiom would be: Jeff should be arriving soon, so keep an eye out for him.
Keep watch on.
it is an old military term for good sight
The image is of you pointing an eye at something or someone. It means to watch carefully.
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.
The cast of Keep Your Eye on the Ball - 1966 includes: Stubby Kaye as Himself - Presenter
It means keep watching the thing that is moving. For example, if they say keep your eye on the ball when you are working, it means concentrate on what you're doing. Keep your eye on the main idea and don't lose sight of your goal.
If you "keep an eye" on anything, that just means you are watching it.
Keep your eye on it and focus