Wallpaper is slang for a security with minimal to no market value.
How Does Wallpaper Work?
Once a security becomes worthless, its hard documentation (for example, its stock certificate) no longer has any practical function. The term "wallpaper" suggests that it be used as such.
Why Does Wallpaper Matter?
Following the Internet startup bust in the early 2000s, the stock of many web-based companies turned into wallpaper in a short span of time. The term doesn't only apply to stocks. For example, the impact of hyperinflation in Germany following World War I resulted in its currency becoming wallpaper.