"Foo" has a long and deep history. There was a comic strip called Smokey Stover that started in 1935 whose title character was a "foo fighter" (firefighter) and drove a "foomobile" (a tiny fire truck with only two wheels). The author of the strip claims to have discovered the word "foo" when he saw it written on the bottom of a Chinese figurine, but I don't think that's the real etymology. I think "foo" was just short for "phooey", and was used to designate things deemed nonserious or trivial (somewhat similar to how "lol-" or "lulz-" is used in modern times).

Whatever the case, "foo" became associated with "bar" via WWII slang "FUBAR"; WWII also gave us "foo fighters" in the UFO sense. War is like the internet in that it brings together people of different backgrounds who otherwise might never interact, so it's not surprising that with things like foo, FUBAR, and Kilroy, WWII had something resembling a predecessor of modern meme culture. It entered hacker culture via the MIT TMRC culture in the 1950s. Their railroad layout had a "foo counter" which was a digital clock which would display the word instead of the time when certain scram switches were pressed. When MIT people began cultivating an interest in programming computers for its own sake, they brought "foo" with them and it spread into the greater hacker meme diaspora.

There's nothing quite as hacker-y as the word "foo", and the strong association with hacker culture goes back as long as hacker culture was a thing, and even before. It still reminds us to not take things so seriously, mess around, and have a little fun.