Adieu is a French word meaning "goodbye" that is commonly used in English, especially in the phrase "I bid you adieu!"
Adieu is one of those borrowed foreign words we’ve tried to make our own. But why would someone say adieu instead of goodbye? Maybe they're trying to be funny in an overly formal way. Maybe they're trying to spice up their vocabulary. Or maybe they're quoting that song from The Sound of Music: "Adieu, adieu, to you and you and you-ooh!" Saying plain old "goodbye" can get boring, so next time try something like "adieu," "adios," "auf wiedersehen," or "sayonara."