back number
Someone or something that is out of date, out of fashion, or no longer relevant. Alludes to an issue of a periodical, magazine, or any serial publication that is now out of date.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.
a back number
If you call someone a back number, you mean that they are no longer useful or successful. This veteran jockey showed he was no back number by coming third out of a field of 24. Note: A back number of a magazine or newspaper is an edition of it that was published some time ago and is not the most recent.
a back number
1 an issue of a periodical before the current one. 2 a person whose ideas or methods are out of date and who is no longer relevant or useful.