An oldie is someone or something that is old, said in a light, often fond way. The most common use is for an old song or an old film that people still like. A golden oldie is a hit from years ago that still gets played. Friends sometimes call a person an oldie too — often warm, sometimes teasing. Though actually, if you do not know the person well, it can sting, because any word that just means “old person” can. Informal. You would not put it in a formal letter.