The Etymology of “Foo”

RFC 3092, baby:

Approximately 212 RFCs so far, starting with RFC 269, contain the
terms `foo', `bar', or `foobar' as metasyntactic variables without
any proper explanation or definition.  This document rectifies that

By extension, there is also “kruft” (which should be well into the general human vocabulary by now) and “kludge” (a technological shunt).