adjective
A word that describes or modifies a noun or pronoun. In this case, 'boringest' is a superlative form of the adjective 'boring', indicating the highest level of boredom.
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The lecture on quantum physics was the boringest presentation I have ever attended.
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That movie was the boringest thing I've ever seen.
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This book is the boringest piece of literature I've come across.
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Sitting through that meeting was like watching paint dry, it was the boringest experience of my life.