As like as chalk and cheese.

The proverb means that two things are markedly different. Its claim that they are alike is ironic: chalk and cheese make a deliberately mismatched pair.

It can also reject a proposed comparison. Rather than identifying a small distinction, it stresses that the resemblance someone sees is unconvincing.

Use it when someone treats two people, objects, or approaches as comparable and you want to emphasize their differences. The wording depends on the listener recognizing the claim of likeness as ironic.