发狠 is a separable verb (离合词) with two main uses. It can mean 'to make a determined effort, to really buckle down' — often with the sense of pushing yourself or others to work harder. It can also mean 'to get angry, to lose your temper' — like suddenly showing anger or hostility. As a split verb, you can insert other elements between the characters, for example: 发了狠 (got angry / made a determined effort), 你发什么狠？(What are you so mad about?). The context determines which meaning applies.