Probably from an earlier*trānsquīlus (withquantitative metathesis), fromtrāns- + the root ofquiēs.

• (Classical Latin)IPA^(key):[traŋˈkʷɪl.lʊs]
• (modern Italianate Ecclesiastical)IPA^(key):[t̪räŋˈkʷil.lus]

tranquillus (femininetranquilla,neutertranquillum,adverbtranquillēortranquillō);first/second-declension adjective

1. (of the weather or similar)quiet,calm,still,tranquil

   Synonyms:misericors,mītis,placidus,quiētus,clēmēns

   Antonyms:obstreperus,clāmātōrius,trux,ferōx,atrōx,silvāticus,violēns,ācer

2. (of a person)placid,composed,untroubled,undisturbed

First/second-declension adjective.

• “tranquillus”, inCharlton T. Lewis and Charles Short (1879)A Latin Dictionary, Oxford: Clarendon Press
• “tranquillus”, inCharlton T. Lewis (1891)An Elementary Latin Dictionary, New York: Harper & Brothers
• tranquillus inGaffiot, Félix (1934)Dictionnaire illustré latin-français, Hachette.
• “tranquillus”, inHarry Thurston Peck, editor (1898),Harper’s Dictionary of Classical Antiquities, New York: Harper & Brothers
• Julius Pokorny (1959),Indogermanisches etymologisches Wörterbuch, in 3 vols, Bern, München: Francke Verlag
• De Vaan, Michiel (2008)Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill,→ISBN,page627