Literally “you surprise me”: tu is “you”, m’ is “me”, and étonnes is “surprise”; the response usually means the opposite of its literal words.
Literally “you surprise me”: tu is “you”, m’ is “me”, and étonnes is “surprise”; the response usually means the opposite of its literal words.
What does tu m'étonnes mean?
Depending on context, “of course” or “no wonder”.
Is the speaker really surprised?
Usually no; the wording is ironic.
Can it answer a question?
Yes, to give an emphatic “of course”.
Is it formal?
No, it is a familiar expression.
Does the headword include an exclamation mark?
No; punctuation depends on the sentence.