さすが, also often appearing in the kanji form
流石（さすが）, is an adverb that indicates the speakers feeling of being impressed that something has lived up to its reputation, or what was expected of it. In English, this comes across as ‘as expected of (A)’, or ‘that is just like (A)’.
As
流石（さすが） is an adverb, it may appear at the beginning of phrases (either with or without に), or directly preceded a noun when の is used.
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すごい！さすが先輩（せんぱい）！
Wow! That’s what I would expect from you senpai!
流石（さすが）に今日（きょう）は雨（あめ）が降（ふ）りすぎだろ...
As one would expect, it is raining way too much today.
さすがの君（きみ）でもこの問題（もんだい）は難（むずか）しいだろう。
Even for someone like you, this problem is difficult, isn’t it?
Caution - Although uncommon,
さすが may occasionally have a negative connotation. In these cases, it is similar to ‘even’ in English. This use of
さすが implies that ‘as one had though, even (A) couldn’t (B)’, and similar nuances.
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さすが田中（たなか）くん！発注（はっちゅう）ミスの天才（てんさい）だね！
As expected, Tanaka-kun! You are a genius at making order errors!
Fun-fact - The kanji structure of 流石（さすが） is a type of kanji use known as 借字（しゃくじ） in Japanese. This basically means ‘borrowed symbol’, and indicates that the meaning of the kanji is not related to the meaning of the word at all.