it'll be better to translate particulary to "Not really"
Anyway the gist of the story is the boy like to say "not really" that it became like his catchphrase
On page 3 the girl tried to imitate him saying "not really"
the boy then rant about his friend said that he said "not really" too much
the boy then realized, and asked if the girl is disgusted(or more like taken aback) by his rant
then the girl said "no-not really"
whatever