The word "毫不" is formed by directly combining the character "毫", which denotes a minute quantity like "a hair" or "the slightest amount", with the character "不", which means "not". This pairing results in a fixed adverbial compound where "毫" figuratively modifies and intensifies the negation, conveying the meaning of "not at all" or "not in the least", as in denying even the smallest degree of something.