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@supermattachine serious answer: the place I would go to for this the linguistics subfield of pragmatics, namely Grice's maxims, and implicatures.
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@supermattachine basically, in conversations, people assume answers will be relevant (& truthful, & other things). so when something is not relevant, you do the gymnastics necessary to convert it into something relevant, and then there's your meaning, shared between participants.
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@supermattachine why do people do that is a hard question, and answering it probably would take you into the murky world of evolutionary psychology.