The expression "went kooky" is an informal way to describe an unusual or erratic behavior. Some synonyms for the phrase could be "went crazy," "went wacky," "went bananas," "went loony," or "went nuts." These expressions denote a person undergoing a sudden outburst of unusual behavior, often in a humorous or quirky way. Other similar phrases could include "went off the deep end," "went bonkers," "went off the rails," or "went berserk." Each of these expressions highlights a unique aspect of irrational behavior, indicating the wide range of emotions and actions that fall under the umbrella of "went kooky".