The word 密探 is formed by combining the character 密, meaning "secret" or "confidential", with the character 探, meaning "to spy" or "to investigate". In this compound, 密 acts as a modifier that specifies the nature of the action described by 探, resulting in the meaning "secret investigation" or, as a noun, "secret agent" or "spy". The formation directly links the concept of secrecy with the act of probing to convey the intended meaning.