Why is the directory for the application source code named "lib"?

I’m curious why the directory for the application’s source code is named “lib” and not, say, “src” or “app.” Does anyone know the history behind that? It’s not a problem or anything, and I’m sure there’s some reasoning behind it. I just find it a bit odd, since “src” or “app” seems to be the established norm in most other stacks.