Superlative form of flimsy; the most thin, fragile, weak, or insubstantial.
From 'flimsy' plus the superlative suffix '-est.' This form represents the extreme end of the spectrum of weakness or insubstantiality.
Using the superlative 'flimsiest' is so common in arguments ('That's the flimsiest excuse I've ever heard!') that we rarely notice we're still echoing 19th-century printer jargon.
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