使性子 is a separable verb (离合词) meaning 'to lose one's temper or act willfully'. The verb 使 and the noun 性子 (temperament) can split when adding a duration, a measure word, or an object between them — e.g. 使什么性子 ('throwing what kind of tantrum?'). It's often used critically when someone is being unreasonably moody or petulant. Do not place an object directly after 使性子; instead, use a 跟 phrase before it if you want to say who is the target of the temper.