The phrase "longest part" refers to the section or aspect of something that is the longest in terms of size, duration, or importance compared to other parts of it. Full definition
With a large chunk of your income no longer part of your budget, would you and the people who depend on you for financial support have enough to make ends meet? (protective.com)
It's also important to have a lot of dialogue with employees about how they can challenge the old ways that are no longer part of success. (entrepreneur.com)
What the proposal would do, is say those parties can no longer part in our marketplace. (utilitydive.com)