A sycophant is a person who praises someone powerful much more than they deserve, in order to get something out of them. It is always an insult. The picture is of someone who follows the boss around, agrees with every opinion, laughs at every joke, and quietly stops saying anything the boss might not like. Two things make a person a sycophant rather than just a polite or loyal person: the praise is not honest, and there is something they want — money, a job, a grade, safety. Notice the direction: sycophants aim upward, at people with power over them. Someone who flatters a weaker person may be dishonest, but we would not usually call them a sycophant. The word is formal and mostly written — journalists, historians and novelists use it about courtiers around a king, advisers around a president, junior staff around a chief executive. Spoken English prefers 'yes-man', 'suck-up', 'brown-noser', 'bootlicker'. It is a noun. Very occasionally you will meet 'sycophant' used as a verb in old or literary writing, but do not do that yourself; the ordinary verb is 'flatter', or 'suck up to'. One more nuance worth having: calling someone a sycophant judges their motive, not just their manners. That is a strong claim to make about a real person, so writers usually save it for cases where the flattery is obvious and the reward is obvious.

馬屁精、諂媚者、應聲蟲。台灣日常最常說「馬屁精」或「拍馬屁的人」；若重點在「老闆說什麼都說好」，說「應聲蟲」更準。注意這是罵人的話，不能拿來自稱或稱讚別人。