In logic, describing an argument that is either invalid (the conclusion doesn't follow from premises) or has at least one false premise. More broadly, it means not based on sound reasoning or reliable evidence.
From the prefix un- (not) plus sound, which comes from Old English sund meaning 'whole, healthy.' In logical contexts, 'sound' arguments are those that are both valid in structure and true in their premises.
An unsound argument is like a house with either a bad foundation (false premises) or poor construction (invalid reasoning) – either flaw makes the whole structure unreliable! Even one false premise makes an otherwise perfect argument unsound.
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