What does "back number" mean?

A back issue of a magazine, etc.

What is a synonym for "back number"?

Common synonyms include: antediluvian, antique, banal, behind the times, bewhiskered, bromidic.

noun

noun ·Uncommon ·College level

Noun

1
A back issue of a magazine, etc.
"Because of war conditions and rigid paper control, it is now impossible for us to keep supplies of recent back numbers."
2
Anything considered old-fashioned.

"Because of war conditions and rigid paper control, it is now impossible for us to keep supplies of recent back numbers."