His boastfulness about getting promoted made his coworkers uncomfortable.

boastfulness 是由动词 boast（自夸、吹嘘）加上后缀 -ful（充满...的）变成形容词 boastful（爱吹嘘的），再加上名词后缀 -ness 构成的名词，表示"爱吹嘘的特性或状态"。