I often hear the "Did not! - Did too!" in TV series and films.
I know perfectly well what it means but why is it the word "too"?
As best as we know "too" stands instead of "indeed/so" and implies an affirmative statement! It's much shorter to use in spoken English!
I believe it is an idiom of some kind but I couldn't find it!
Brother: Mom, Linda kissed a boy at school today!
Sister: Did not!
Brother: Did too!