The loud, powerful sound an engine makes, especially when a car accelerates quickly.
An onomatopoeia created in the early 20th century to imitate the sound of automobile engines. It became standard in comic books and later in casual speech to represent engine noise.
Vroom is pure sound-spelling—comic book creators invented it to make readers *hear* speed on a silent page, and it worked so well that actual car enthusiasts adopted it into real conversation.
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