Giving very good oral sexual favors. The best from that particular individual.
She gave me sex all night and Fly Head in the morning.
A 'fly head' is used to describe someone's testicles that are bigger than their penis. The testicles represent the fly's large eyes, and the penis represents the thin/small mouth of the fly, thus being called a 'fly head'.
person#1: Haha look at James, his balls are so fucking big it looks like he has a camel toe.
person #2: Yeah, i reckon it looks a bit like a fly head.