“Back number” is an English idiom referring to something or someone that is totally out of fashion with current trends or practices. This idiom can refer to a person who has not kept up with the way things are currently done and thus seems out of place in a crowd. It can also refer to an item of clothing or anything else that may be out of date compared to other of its kind. Originally, “back number” referred specifically to periodical issues from dates previous to the current time, but it has come to refer to anything that is out of date.
In English, an idiom is a word or short phrase that is used by speakers as a means of providing some color or spice to their conversations. The unique thing about an idiom is that it may mean something entirely different than what someone might expect from the literal definitions of the words. Instead, idioms take their meanings from the way they are used and understood in a specific culture. One idiom that is used to indicate something that is old-fashioned is the phrase “back number.”
If a person is described in this manner, it means that he or she has not kept up with the times in some manner. While the phrase often refers to fashion in terms of clothes, it can be used to describe a person who has kept some outdated habits or methods. The phrase can either be used as a gentle jibe or a searing insult. As an example, someone might say, “Look at that back number with her silly hat that looks like it came from her grandmother.”
In addition to describing people in this manner, this phrase is flexible enough to be used on non-living thing. Anything that has outlived its trendiness or fashionableness can be referred with this phrase. For example, consider the sentence, “I can’t believe he is still using that ancient desktop computer; it's a real back number.”
The phrase gets its meaning from the fact that a “back number” of a periodical like a magazine or a newspaper is an issue from some past date. Much as the news or information contained in such old issues wouldn’t be current, so too does the idiom refer to someone or something who has not kept up with the times. As is the case with many idioms, the phrase evolved from its literal meaning to take on a more figurative tone.