Is this racist?

Just because you've never seen a Pakistani cloakroom attendant it doesn't mean that they don't exist and doesn't make it any less racist to make that joke, even if none existed at all.

But you were inferring that the joke was racist because it was aimed at the stereotype of Pakistanis working as CR attendants so I was making a point

After all, why is the same joke not about all cloakroom attendants?

Because the silly punchline of Ma Hat Ma Coat sounds like a Pakistani name(for the purpose of the punchline) The joke is stupid and meaningless but definitely not racist

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