a despised person. (This expression is used in place of other very insulting terms. Often modified, as in the examples.) You dirty so-and-so! I can't stand you! Don't you call me a so-and-so, you creep!
so-and-so
n. a euphemistic way of addressing someone with an insulting term of address. (This is usually euphemistic for son of a bitch. However, it is also used for good friends, especially male to male, to show affection.) This lousy so-and-so tried to take me for 400 bucks.