Boastfulness is the noun: the habit or quality of talking too proudly about yourself. We use it when we want to name the thing itself, not how someone is speaking in this one moment. A person full of boastfulness keeps pointing to their own wins and leaves little room for anyone else. It is a little formal. A friend is more likely to say He's always showing off than His boastfulness is a problem. Teachers and writers like the noun because they can talk about the quality, the same way they talk about kindness or laziness. Though actually, a little pride in good work is not the same as boastfulness — boastfulness is the extra, the part that pushes other people down.

自誇、愛吹噓（指這種習氣，不是某一句話）