An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
put (something) on paper
to write something down
I put my ideas for the new business on paper.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is a group of words, or in other words, a phrase that has a meaning different from the literal meaning of the words in it.