扣帽子 is a separable verb (离合词) meaning 'to label someone unfairly' or 'to pin a political/negative label on someone'. It comes from the literal image of 'putting a hat on someone' — you're forcing a label onto them that they don't deserve. The structure splits: 扣 can take a modifier or object, and 帽子 can be modified. You'll often hear it in political or workplace contexts when someone is accused of a vague crime or ideology without evidence.