As an acronym, it may stand for "Battlefield Identification Friend or Foe", which I imagine to be some form of electronic military means of establishing who forces in a war-zone are. It may also be "Binary Interchange File Format", which is presumably some kind of computer jargon the meaning of which I personally couldn't even begin to guess at! As a 'word', it is an onomatopoeia - ie derived from its own sound - as Wendy says.
I love that word - Onomatopoeia - i'm sure it's what the girl is chanting at the beginning of the Michael Jackson track "Liberian Girl". (THUMP) Ok, i can take a hint. Oh, that was another example of an onomatopoeic word...
When I was at school in the late sixties the headmaster used to dish out "biffs" to us across the palms of our hand with a leather strap if we hadn't done our latin homework properly 3 for lack of memory or 6 if he thought it was premediated.