An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
go off (somewhere)
to leave or depart for somewhere
My friend went off on a trip and did not say good-bye.
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.