Derived from George Orwell's novel 1984 from 1949 in which the 'Big Brother' is a theme that stands for the absolute surveillance and monitoring of people by their own government.
Nowadays one can use it to tell some one to be careful what he or she does, because he or she is always being watched (most of the time by the one that tells him BBiwy).
Don't even try to meet Janet. I'll find out anyway. BBiwy.