The length of time it takes for a man to move the tip of his penis one mile in the reference frame of the vagina while having intercourse.
The following formula can be used for a simple calculation: (63,360 inches / 2*(penile length)) * (Average thrust time), where penile length is in inches. The units of average thrust time will determine the units of the fuckmile.
Man, I was waiting at the DMV for like 3 fuckmiles yesterday.
Guy: Threesome?
Guy: With him?
Guy: I wouldn't have a threesome with him even for a billion dollars.
Guy: Maybe a trillion, just maybe.
Girl: What about a fuckmillion?