how so?
From Longman Dictionary of Contemporary Englishhow so?how so?OPINIONused to ask someone to explain an opinion they have given ‘Rick’s parents are a little strange, I think.’ ‘How so?’ → howExamples from the Corpushow so?• "The scriptures make it perfectly clear." "Oh, how so?"• Have you noticed how so many adverts for chocolate are aimed at girls?• I do not understand how so many Black men stay sane, look after their children and strive to walk beside them.• He does not entirely explain how so many people could lift off so quickly into the deal-making stratosphere.