As in, is that so? If yes, then is that so is another way of saying, is that the case/true? Elliptical That so?
as a term of dismissal is attested from 1886 (short for is that so?)
so3 [səu] adverb
(used in place of a word, phrase etc previously used, or something previously stated) as already indicated
Example: `Are you really leaving your job?' `Yes, I've already told you / said so'; `Is she arriving tomorrow?' `Yes, I hope so'; If you haven't read the notice, please do so now; `Is that so (= true)?' `Yes, it's really so';
so - Definitions from Dictionary.com
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