A transition from napping/cuddling to sensual foreplay, followed by passionate, (rough) sex... aka, I really want to hold you and be close, but let's go to pound town on the fuck truck.
Simply put, it's a cuddle that turns into a fuck. And it's really good. Also referred to as SF, or SFing.
I'm going to snugglefuck the shit outta you!
I wanted to fuck her really bad, but I also wanted to hold her in my arms....so I settled for a snugglefuck.