The most foolish, ridiculous, or not serious in the most extreme way.
From 'silly,' which comes from Old English 'selig' meaning 'blessed' or 'innocent.' Over time it shifted to mean 'simple-minded,' then 'foolish,' with the superlative form 'silliest' being the most extreme version.
The word 'silly' used to mean something positive—'blessed' or 'innocent'—but words can completely flip their meaning over centuries. This semantic flip happened because innocent people were often seen as lacking in street smarts.
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