8 letters · 17 points in Scrabble
adjective (of a body of water) free from disturbance by heavy waves
adjective not agitated; without losing self-possession
originBorrowed from Middle French tranquille, from Latin tranquillus, from trāns- + the root of quiēs (“rest, quiet, peace”), ultimately from *kʷyeh₁- (“to rest”).
Etymology from Wiktionary, CC BY-SA 3.0