The characters combine to mean “On behalf of the tiger, act as a Chāng ghost.” This literal image creates the powerful figurative meaning: to actively guide a villain to their prey, becoming a willing or unwilling tool for evil.
The soul of this idiom lies in the tragic myth of the 伥鬼 (chāngguǐ). Unlike a simple accomplice, the Chāng ghost is itself a victim. This adds a layer of complexity and tragedy. The ghost, having suffered the worst possible fate, is now forced to inflict that same fate on others. This story makes the act of `为虎作伥` particularly heinous in Chinese culture because it represents the ultimate betrayal—a victim helping the victimizer perpetuate the cycle of violence. A Western parallel might be a “henchman” or “collaborator,” but these terms lack the supernatural and tragic backstory. For instance, “aiding and abetting” is a legal term, while `为虎作伥` is a moral condemnation rooted in folklore. The idiom doesn't just describe an action; it evokes a specific, culturally understood image of a cursed spirit leading a monster. This highlights a deep cultural value placed on loyalty to one's community and the abhorrence of siding with an oppressor, especially against your own kind.
`为虎作伥` is a formal and very strong idiom. You won't hear it in casual daily chatter, but it's common in more serious contexts. It always carries a heavy negative connotation.