"Now and again" is the same as once in awhile.
It's not an idiom. It's just a sentence -- to be young again means that you are once more younger in age. This can be literally -- like in a science fiction time travel story -- or it can be figurative -- like your mind is young again.
i don't now
This is the equivalent of our American idiom: If you fail, try try again.
It is an idiom meaning that someone who had stopped drinking alcohol has slipped up and started drinking again.
It means "sit down" in idiom slang. The original phrase was "pop a squat", the word "cop" is a dialect misunderstanding, but now both pop and cop are used in the idiom.
It's not really an idiom. It means "what are you thinking about."
It is not an idiom, it means your nose is itching.
RFP is not an idiom. It's an abbreviation.
It means "do you understand" in an aggressive and standoffish manner. As in "If you show your face here ever again, you will cease to exist... you dig it!"
"Sieve" is not an idiom. See the related link.
It's not an idiom. It means the tip of your nostril.
idiom means expression like a page in a book