Act like locals. An outsider who takes on the mannerisms of locals. ‘Native’ is used in the sense of inhabitants of some distant island wherein someone enjoys the life style and customs of the inhabitants. Often used in the negative to imply that someone is acting like a local which is less civilized than they were before.
“Now that he became a manager, he has gone native and treats the rest of us, his former associates, like trash just like all other managers treat us.”