The word "谁知" is formed by directly combining the character "谁", meaning "who", with the character "知", meaning "to know", creating a subject-verb construction where "谁" serves as the subject and "知" as the verb, resulting in the literal phrase "who knows". This compound functions as a fixed lexical unit used to express surprise or pose a rhetorical question, derived from the straightforward syntactic pairing of its components.