[be|become] full of {oneself}
1
to be extremely proud and believe that one is superior in intellect, importance, etc.
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.
- Ever since he got that promotion, he's been so full of himself that it's hard to have a conversation with him.
- He's been acting like he knows everything about the topic, but in reality, he's just full of himself.
- I used to be friends with her, but she became so full of herself that I couldn't stand being around her anymore.
- She's always talking about her accomplishments and how great she is; she's really full of herself.