a 'so and so'

Sometimes a person is described as a 'so and so'. What does this mean and from where was it derived. I understand a 'so and so' is not a compliment.

It's an insult because "so-and-so" is a deliberately vague euphemism, free of content, that replaces whatever insulting term the speaker is supposedly suppressing. The idea is "I'm too polite to use the words he deserves, so I'll call him a "so-and-so" instead." ~rb

Bergie is absolutely right. However, you may find the "insult" is sometimes a friendly one, as at, say, a reunion of old friends. "Well, if it isn't Joe Blow! How are you, you old so-and-so?"
SS