Halal cheating

It is when a Muslim man gets a secret wife without the permission of his first wife (In Islam, polygyny is allowed). In moralistic terms, this is considered cheating, but it is considered "permissible" in Islam. Hence, you may cheat without the permission of your first wife, as long as you're doing it the halal way!

Married woman: I just learned that my husband had another wife behind my back for 12 years! I feel so crushed. Was I not enough?
Sister: *sighs* Poor woman, you just caught him halal cheating.