Simple past tense and past participle of dull.(verb)
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Use "dulled" in a sentence
"A pained expression dulled his features, and he shook his head."
"On the battlements above, Jonat - helmet gone, sword dulled - watched helplessly as the Vagrians swept over the ramparts."
"He has documented his experimental research which has demonstrated that the "executive function" of the brain – the part that controls thinking, problem solving and task completion – has been significantly affected, and 'dulled' – by exposure to electronic media."