So-and-so is used as a stand-in for a name that you aren't sure of or in place of using a vulgar expression. (noun)
An example of when you might use the expression "so-and-so" is when someone at a party told you that the stock market was crashing but you don't remember her name. You might say "so-and-so said the stock market is crashing."
An example of so-and-so is when you call someone a stupid so-and-so.
Informal some person or thing whose name is either not specified or not known: often used euphemistically in place of a strong or vulgar, disparaging epithet