毫不迟疑 (háo bù chí yí) is a fixed adverbial phrase meaning 'without the slightest hesitation'. It is formed from 毫不 ('not at all') and 迟疑 ('to hesitate'). You use it before a verb to describe an action done decisively and immediately, without pausing to think. It is common in formal or literary contexts. When used as an adverbial, you can add 地 or use it directly before the verb.