expression
Used with a negative verb to emphasize that not a single person does or is something. Stronger than 誰も〜ない.
誰一人としてその質問に答えられなかった。
Not a single person could answer the question.
彼の意見に誰一人賛成しなかった。
No one agreed with his opinion.
誰も〜ない is the standard 'no one' pattern. 誰一人〜ない adds stronger emphasis on 'not even one person'.
Combination of 誰 (who) and 一人 (one person), used with a negative to mean 'not even one person'.