He keeps brown-nosing the boss to get a promotion, which is really annoying.

brown-nosing 是一个复合词，由 brown（棕色）和 nose（鼻子）组成。字面意思是"棕色的鼻子"，比喻那些过分讨好、谄媚上司的人，就像狗讨好主人时鼻子会变脏一样。这个短语带有贬义，指"拍马屁、阿谀奉承"的行为。