Someone or something that is out of date, out of fashion, or no longer relevant. Alludes to an issue of a periodical, magazine, or any serial publication that is now out of date. Primarily heard in US, South Africa. While it created quite a controversy in its time, the movie's love scene is a bit of a back number by today's standard. The singer was a huge star in the 1980s, but she's just a back number now.