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1 an issue of a periodical before the current one. 2 a person whose ideas or methods are out of date and who is no longer relevant or useful.