tranquillize v make calm or still v cause to be calm or quiet as by administering a sedative to Usage(s) It worked to tranquillize strikes and lockouts affecting 350,000 workers (but claimed no great success in the bitter, long-drawn coal strike of last winter, which it proved . Synonym(s) calm calm down quiet tranquilize tranquillise quieten lull still sedate Antonym(s) agitate stimulate