When a girl gets tired of fucking the same cock, even though that cock might be awesome by size, girth, cumshot.
Girl #1: I cheated on Danny.
Girl #2: Why? I thought you said he had an incredible cock...
Girl #1: It's been 3 months, I'm tired of his cock, so I did a one-niter with this guy I met on Twitter.
Girl #2: Sounds like you have cock fatigue.
Girl #1: Yeah you could call it that. I'm just afraid to tell Danny, don't want to deflate his ego. There's nothing wrong with his cock. I just need fresh new cock!