Superlative form of serene: most serene.(adjective)
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Use "serenest" in a sentence
"I find something – like language – immaterial, but earthly, terrestrial, something circular, something that traverses both poles to return into itself and there – in the serenest of ways – even intersecting the tropics - I find ... a meridian."
"“We shall have the serenest & happiest time while you are here, & nobody shall know care or fatigure.”"
"“That conquers the serenest of them,” Mark Twain noted with evil satisfaction."