If something is under your nose, you'd see it, right? It means that something is right there, in plain sight, obvious to everyone.
Sometimes we are looking for something 10 or 100 feet away and it is right at our feet and don't see it, hence, 'right "under your nose"'.Something that is under someone's nose is something that the person is seeking that is right in front of him/her.
To poke your nose into is to insert yourself into someone else's business.
It is not an idiom, it means your nose is itching.
Yes it is an idiom because the literal meaning doesn't make sense.
An idiom is a phrase that makes no sense unless you know the definition. Can a nose actually run somewhere? No, so this is an idiom.
to ignore something
to hide something and not talk about it.
A British police euphemism meaning to keep out of trouble, commit no crimes
"The idiom 'that just kills' is hardly appropriate at a funeral."
"To be" is not an idiom - it's a verb.
say no to it
Pest is not an idiom. It's a word.