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 in its intensive sense of "exceedingly") + a root possibly related to quies "rest" (see quiet (n.)).