The superlative form of dull; the most dull, boring, unintelligent, or not sharp among all things being compared.
From 'dull' + the superlative suffix '-est' (the highest degree, as in 'biggest,' 'fastest'). This is the standard way to express the extreme degree of dullness in English.
The dullest thing is almost funny—when something is so boring or stupid that it reaches a peak level, it becomes almost notable for how completely unremarkable it is!
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