An idiom is a widely used saying or expression containing a figurative meaning that differs from the phrase's literal meaning.
second nature to (someone)
to be easy and natural for someone
Playing a musical instrument is second nature to my friend.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is defined as - a group of words in a fixed order that has a particular meaning that is different from the meanings of each word on its own.