If someone is swellheaded, they think they are very important or better than other people.
Being swellheaded is not a good thing. It can make people not like you.
Examples:
He is so swellheaded, he thinks he is the best at everything.
She doesn't like to play with him because he is too swellheaded.
The swellheaded boy always wants to be first in line.
The cat is not swellheaded, it just wants to sleep all day.
The king was so swellheaded, he thought he could do anything.
The swellheaded girl always talks about how good she is at everything.