Here are some sentences for this idiom.
keep your chin up!
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.
Come on friends, things will get better! Just KEEP YOUR CHIN UP and you'll see!!
He tried to keep a straight face although he thought what she had done was very silly and stupid.
"They will try to make you mad, but keep your shirt on while I make the deal." "The waitress told the impatient customer to keep his shirt on, and that he would be served when it was his turn."
Keep watch on.
A chin guard is mainly there to keep your helmet on.
To keep your chin up is an expression to maintain positive outlook.
A memento is, essentially, a souvenir. An example sentence would be: She wanted to keep the memento from her trip.
Example sentence - The scarecrow stands in the garden to help keep birds away.
This is not an idiom - it means exactly what it says. You should stay fit and healthy.
It is just an idiom and has no history.