Pride
Boast
noun, する verb, の adjective
You ridicule everyone but yourself because you have so much pride. When you talk about yourself you only boast because there's nothing to ridicule. That's how great you think you are.
じまん
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
自分の不倫を得意気に自慢するとか、ありえないんだけど。
It’s impossible for me to proudly boast about my own affair.