The line 'Achtung baby' comes from the 1966 Gene Wilder film The Producers. In the film a pair of Broadway directors put on Springtime for Hitler - a musical intended to be a flop so as to effect a tax dodge. Dick Shaw's character, a spaced-out hippy actor playing Hitler, says the memorable line.

It sounds cute and memorable because people tend not to think that such a frivolous word like 'baby' could ever be said by a German.

Mel Brooks would again later use this phrase, and others from the film like "Don't be stupid be a smartie, come and join the Nazi Party" in his 1983 single To be or Not to be. Unlike Springtime for Hitler his single flopped, and not by design.

To all the little mothers in the Fatherland
I said "Achtung baby! I got me a plan."
They said "What you got, Adolf? What you gonna do?"
I said "How about this one? World War Two."

Subsequently a U2 crewmember liked using the phrase and it stuck with the group. It appears in the lyrics of The Fly. And the album did not flop.