毫发 is a literary expression that literally means 'a hair' but is used figuratively to mean 'the slightest bit' or 'a hair's breadth'. It almost always appears in negative constructions such as 毫发无损 ('not harmed in the slightest') or 毫发不差 ('not differing by a hair'), emphasizing that something is completely unharmed or exactly the same. Note that 发 is pronounced fà here (the literary reading for 'hair'), not fā.