No, the word "feeling" is not an idiom. It is a noun that refers to an emotional state or perception of a situation.
The meaning of the idiom dead certainty is absolutely or definitely
"She's feeling under the weather" is an example of an idiom, meaning that someone is feeling unwell or sick.
In one word: dead.
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This expression's meaning may be guessed from the words in it, so it is not an idiom. To play possum is to deceive an attacker by pretending to be dead or vanquished, as the possum proverbially does.
An idiom is a phrase that makes no sense unless you know the definition. "Feeling" is a word.
The idiom "my heart was in my mouth" means feeling extremely anxious or nervous to the point where it feels like your heart is pounding heavily in your chest.
It's that feeling that something is in your throat, keeping you from swallowing. You often get this feeling when you become very emotional, such as very sad or very happy.
"Feeling blue" IS an idiom - you cannot literally feel the color blue! Feeling blue means that you're sad.
An idiom is a phrase or expression that has a meaning different from the literal definition of the individual words. Idioms are often used to convey a specific message or feeling in a culturally understood way.
To nurse a grudge is to have a long lasting feeling of resentment over a slight, or insult, etc, someone did to you.