Is the phrase living out of a backpack an idiom?

"Living out of a backpack" is not a true idiom. It's a literal description, meaning someone is carrying all their belongings in a backpack and traveling.

However, it's often used figuratively to describe a nomadic lifestyle or someone who is unattached and free-spirited. In this sense, it becomes a metaphor for a lifestyle choice.

So, while it's not a traditional idiom with a fixed meaning, it's a commonly used phrase that can evoke certain connotations and imagery.