The phrase metasyntactic variable (pronounced /ˌmɛtəsɪnˈtæktɪk ˈvɛəriəbl/) is a neologism that is used in some programmer communities to describe a placeholder name or an alias term commonly used to denote the subject matter under discussion or an arbitrary member of a class of things under discussion. The term originates from computer programming and other technical contexts, and is commonly used in examples by hackers and programmers. The use of a metasyntactic variable is helpful in freeing a programmer from creating a logically named variable, although the invented term may also become sufficiently popular and enter the language as a neologism. For example, in Terry Pratchett's book Men at Arms, the dog Gaspode says, "Clothing has never been what you might call a thingy of dog wossname", and then adds, "Two metasyntactic variables there. Sorry." The word foo is the principal example.Wikipedia: where the simplest ideas are described by the clumsiest definitions (and a reference to Terry Pratchett).
Here is a cartoon called ASDF; it was made by a PhD student in the Low Countries, and was made known to me by Cass Goodwin.