Its origin is unclear, but it has been in use since the mid-19th century. It is thought to have developed from the idea of being filled up with one's own self-importance, leading to an inflated sense of ego. Some believe the phrase may have originated from the image of someone literally being filled with themselves, such as an actor who has a high opinion of their own performance. Regardless of its origins, "be full of oneself" remains a common expression in modern English.

The origin and history of "be full of oneself"

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to [be|become] full of  {oneself}

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Definition und Bedeutung von „ be full of oneself“ im Englischen

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