johnny-come-lately

Example: “The old members resent the johnny-come-lately who wants to change the rules.”

Origin: American and nineteenth-century, built on an older British service term. "Johnny Newcome" was naval and military slang of the early 1800s for a raw recruit or a newly arrived officer, and it relied on Johnny as English's all-purpose name for an anonymous ordinary man — the same generic use behind Johnny Reb, Johnny-on-the-spot, and stage-door Johnny. The American reshaping keeps the function and adds a rhythm. What both forms encode is a specific social judgment rather than a neutral observation about timing: the newcomer's offense is not arriving late but presuming on a standing that the people already present consider unearned.