A saddo is a person you do not respect, especially because you find their life or their hobbies boring, lonely, or strange. It is British informal talk, a bit unkind — the sort of word people use to tease or to look down on someone. “He stays in and sorts his tickets every Saturday. What a saddo.” Though actually, the speaker is often being snobbish: the “boring” interest may just be one they do not share. The sad here is slang for “pathetic,” not tears, and the word has nothing to do with a horse’s saddle. You would not use it in a formal essay, and you would not say it to someone’s face unless you meant to sting.