Have kittens is an idiomatic expression that means to get very upset, anxious or nervous about something. There are several synonyms you can use instead of have kittens, such as have a conniption, have a cow, freak out, go berserk, go ballistic, go nuts, lose your composure, or have a meltdown. These terms all convey a sense of intense emotional distress, usually resulting from a sudden shock or unexpected news. Using these different expressions can help to spice up your writing, avoid repetition, and add a bit of color to your language.