Stands for ball stuck to leg, or balls stuck to leg. This condition commonly occurs in humid climates when wearing loose-fitting underwear, such as boxer shorts.
When the air is very humid and the scrotum's sweat glands are active, muculent fluids will sometimes cause the scrotum to collate itself to the leg or inner thigh.
Man, I really need to get out of this class and have a shower. I have some serious BSTL going on down here.