oliveone wrote on Dec 17^th, 2010 at 7:11pm:

The first time I heard BUF, I was in Thailand, 1969, pre-flighting a B52D. I was calling the plane "Bongo52" when a crewmate asked if I had heard of BUF. At that time, and for the rest of my two combat tours, BUF stood for Big Ugly F***er. Never heard the term BUFF until the 1990's. Don't know where/when the second F was added. Cheers

Same, the aircraft itself is actually looks more like a pencil with wings, so why call it fat?  Makes no sense to me.