The phrase "went cuckoo" is a colloquialism that means someone has gone insane, crazy, or daft. There are several synonyms for this phrase that can be used to convey the same meaning or sentiment. For instance, one can say someone has gone bonkers, bananas, or batshit crazy. Alternatively, one can use the expression off their rocker, out of their mind, or lost their marbles. Other synonyms include crackers, nuts, or loony. These idioms can spice up one's writing or speech, adding a creative flair to the language and portraying a vivid picture to the reader or listener.