When someone "goes cuckoo," it means they have lost their mind or become irrational. There are many synonyms for this phrase, including going crazy, losing it, flipping out, having a breakdown, going bananas, and being out of their mind. Other expressions that can be used include becoming unhinged, having a meltdown, going bonkers, losing one's marbles, going off the deep end, and being certifiable. These phrases can be used interchangeably to convey the same message when someone's behavior has become erratic or irrational. Overall, "going cuckoo" is an expressive and colorful way to describe such behavior.