He is so conceited that he believes he's the best at everything.

conceited 是形容词，由动词 conceit（自负）加上后缀 -ed 构成。conceit 来自拉丁语 concepere（构想），引申为"自以为是、自负"的意思。conceited 形容一个人过度高估自己、自视甚高。