Superficially, he is impressive, but his knowledge is really very slight.

superficially 是由形容词 superficial（表面的、肤浅的）加上后缀 -ly 构成的副词形式。superficial 本身由前缀 super-（在...之上）和词根 facies（表面）组成，字面意思是"在表面之上的"。superficially 表示"表面上、从表面看"的意思。