What does 流石 Sasuga mean?

Sasuga is admiration expressed as confirmation. Somebody has done something impressive, and the speaker's response is that this is precisely what that person was expected to do.

Praise without surprise

English praise usually contains surprise. Well done implies an outcome that might not have arrived; impressive suggests a bar that has been unexpectedly cleared. Praise that removes the surprise tends to become sarcastic in English, since of course you did carries an edge almost automatically.

This word does the opposite. Removing the surprise is the compliment, because it locates the achievement in the person's established quality rather than in a good day.

What it appeals to

Reputation. The speaker is invoking a shared understanding of what this person is, and confirming that the evidence has arrived on schedule. It is a social act as much as an evaluative one: it tells everyone present that the reputation is agreed.

Which is why it lands so well after a difficult thing done easily, and why it would be strange after a lucky result.

The writing

The characters used for it were chosen for their sound rather than their meaning, which is a common enough arrangement in Japanese and produces written forms that cannot be parsed. Anyone attempting to decode this one by combining the senses of the characters is wasting their time; the spelling is a phonetic costume.

Worth knowing generally, since it is a standing trap when reading Japanese: not every written word is a compound to be taken apart.

The subtitle problem

As expected reads flat or arch. Of course reads sarcastic. That's our man is close in spirit and far too colloquial for most contexts.

So the line is usually rendered with something that loses the mechanism, and English viewers absorb it as generic praise, missing that the character has said something specific about the other person's established character rather than about the moment.

The register

Warm, common, and usable upward, which is unusual for praise. Complimenting a superior is delicate in most languages, and this manages it by not claiming to judge them: it merely confirms what everyone already knew, which is a safer thing to say to a person above you.