Simple past tense and past participle of frown.(verb)
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Use "frowned" in a sentence
"Quintana glanced at the Saint again; but the Saint, who understood every word as easily as if it had been spoken in English, frowned back at him with the worried expression of a man who is trying hard to understand and failing in the attempt."
"Martin frowned inwardly at the citation of that worthy gentleman, and went on: I put stamps on all my manuscripts and started them off to the editors again."
"In saner times, "prove it's not true" would be a phrase frowned on in an innocent until proven guilty culture."