mid-15c., a back-formation from tranquility or else from Latin tranquillus "quiet, calm, still." Related: Tranquilly.
Entries linking to tranquil
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.
tranquilize v.
1620s, from tranquil + -ize. Related: Tranquilized; tranquilizing; tranquilization.