The word 不为人知 is formed by sequentially combining the characters 不 (not), 为 (acting as a passive marker meaning 'by'), 人 (people), and 知 (to know). This structure creates a passive phrase where 不 negates the verb 知, 为 introduces the agent 人, resulting in the literal meaning "not known by people", which conveys the idea of being unknown or obscure.