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The expression "Some of my best friends are..." dates from the 1940s or earlier, and was often used to defend oneself from being seen as prejudice. The phrase is completed by the group of people the person is being accused of being prejudice against. For example, someone who is being accused of being anti-Semitic would say, "But some of my best friends are Jewish." The expression used to be taken seriously, but due to overuse, it has become unconvincing and is often used sarcastically.
TVTropes. (n.d.). Some of my best friends are x. https://tvtropes.org/pmwiki/pmwiki.php/Main/SomeOfMyBestFriendsAreX