Tranquility

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late Middle English: from French tranquille or Latin tranquillus .

wiktionary

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tranquil +‎  -ity, from Middle English tranquillite, from Old French tranquillite, from Latin tranquilitas. 

etymonline

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tranquility (n.)

also tranquillity, late 14c., from Old French tranquilite "peace, happiness" (12c.), from Latin tranquillitatem (nominative tranquillitas) "quietness, stillness; serenity," from tranquillus "quiet, calm, still," perhaps from trans- "over" (here perhaps in its intensive sense of "exceedingly") and an adjective from PIE root *kweie- "be quiet," but de Vaan finds this "semantically vague" and phonetically disputable.