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.