“乱发脾气”常见的英文表达有 “lose one's temper unreasonably” 、“fly into a rage for no good reason” 或 “get angry without justification” 。
lose one's temper unreasonably： “lose one's temper” 是 “发脾气” 的常用表达，“unreasonably” 表示 “不合理地、无理地”，整体意思就是 “无理地发脾气，乱发脾气” 。例如：He often loses his temper unreasonably, which makes his friends stay away from him.（他经常乱发脾气，这让他的朋友们都远离了他。）
fly into a rage for no good reason： “fly into a rage” 是一个固定短语，意思是 “勃然大怒；突然发火” ，“for no good reason” 表示 “没有正当理由”，合起来就是 “毫无理由地突然发火，乱发脾气” 。例如：She flew into a rage for no good reason and started shouting at everyone.（她毫无理由地乱发脾气，开始对每个人大喊大叫。）
get angry without justification： “get angry” 是 “生气、发怒” 的意思，“without justification” 表示 “没有正当理由” ，整体表达 “没有正当理由地生气，也就是乱发脾气” 。例如：It's not right to get angry without justification just because of a small thing.（因为一件小事就乱发脾气是不对的。 ）