The phrase "cock a doodle doo" is often associated with the sound a rooster makes to signify the start of a new day. Its antonym would be a noise that signifies the end of the day, such as "goodnight" or "sleep tight." Other antonyms for "cock a doodle doo" could be sounds that indicate silence, such as "shh" or "hush." Conversely, a synonym for "cock a doodle doo" could be "crowing," or other animal sounds such as "barking" or "meowing." These words carry different meanings and nuances, showcasing the versatility of the English language.