"Keep your hair on" could refer to the Old West, when Indians might scalp you if you were not watchful. I've never heard it said as "keep your hair on" though.
I suppose the idiom 'keep your hair on' means that if when a person is stressed the likelyhood that you could lose your hair or even pull it out hence keep it on and keep yourself calm
This may be a mixed idiom - more common is "keep your hat on" which is also means keep calm and don't "blow your top".
"Keep your hair on" is advice telling someone to keep calm and not to over-react or get angry.
This is not an idiom - it means exactly what it says. You should stay fit and healthy.
Stay calm
you was close to something,you almost got it
It could mean a large roll of hair on the back or top of the head, or it could mean a large buttocks or it could just mean a large bread roll.
It's not an idiom that I'm familiar with. It sounds like a description - something is either literally 3 miles long and smaller than a human hair in diameter, or it is an exaggeration of something being very long and thin.
Keep bothering someone.
Quit bugging you-an expression.
It's not an idiom because you can figure it out by the context. It means they went bald.
This is not an idiom - it means exactly what it says. You should stay fit and healthy.
To keep struggling and not give up.
Stay calm
it means that
you was close to something,you almost got it
Doing anything "by a hair" means that you barely manage it. If you win by a hair, you win only by the margin as wide as a hair is. If you lose by a hair, you almost won. You can also have a "close shave," which is an idiom meaning that you escaped something bad by a hair's width.
What happens when something gets in your hair? It's annoying, right? You have to stop and deal with it. Getting in your hair means interrupting you, getting in your way in an annoying fashion. A similar idiom is getting on your nerves.
If your head is "in the clouds," you're a dreamer.
It means " keep calm, don't get worked up"