as one would expect; as is to be expected
さすが expresses that something is as expected, often praising or acknowledging someone or something that lives up to their reputation.
さすが is a common expression meaning 'as one would expect.' It is used when someone or something meets your expectations, often in a positive way. For example, if a friend who is good at cooking makes a delicious meal, you can say さすが! to mean 'just as I expected from you!'
さすが can be followed by に to form an adverb meaning 'as expected' or 'certainly,' often used when acknowledging a limitation or a fact: さすがに疲れた means 'I'm really tired (as expected).' When used with の + noun + も, like さすがの先生も間違える, it means 'even someone as good as the teacher makes mistakes.'
It is a very common spoken expression and is fine in both casual and polite conversation. In writing, it's also acceptable but more often seen in dialogue or informal contexts.