I think it derives from the BUFF, Big Ugly Fat Fellow - the 747, and the C-5A.
I've more often heard SLUFF, Slow Little Ugly Fat Fellow. The first 25, the 737-100, were even shorter than the -200, I believe.
Then, in honor of the Ford Tri-Motor, there's the Boeing Tri-Motor, 727, and the Douglas Tri-Motor, DC-10.
I've heard the MD-80 called the Pecker.
GB