Acronym for "Be Careful What You Ask For (because you may get it)". This is what you might say to someone who wants something, but has not yet thought about the "unintended consequences". For example, someone might have said "I sure wish we had a president with a "W" for a middle initial!" BCWYAF!
note: this is pronounced "be careful..." and said with a trailing note that the rest of the sentence is un-said.
I sure wish we had a woman, or a black, for a president.