Idioms – Meaning of ‘To Think’ Idiom

Oxford explains it:

Think nothing of - Consider (an activity others regard as unusual, wrong, or difficult) as straightforward or normal.

In other words, when you do something unusual without thinking of others, you do it easily. The FreeDictionary explains it.

It basically means exactly what you say that's super obvious or you don't need to even say that.

"The square root of 4 is 2"
"You don't say!"

Another one people often use to say the same thing would be calling someone captain obvious.

"The square root of 4 is 2"
"Thanks Captain Obvious!"

A site to read for a good definition of this: You don't say in the The Free Dictionary

Look at the second definition: 2.Inf. You have just said something that everybody already knows.

The first definition you might here though is the non-sarcastic one, which is a genuinely surprised response to something someone said.