A memento from a good blowjob. A blowmento is when you're taking a piss the day after getting a full service BJ and you find one of the girl's extra long hairs stuck somewhere in your package.
Guy 1: Dude, check it out. I found this blowmento wrapped around my wiener while I was taking a piss.
Guy 2: What's a blowmento? That's a piece of woman hair.
Guy 1: It's a hair from that broad I met at the bar last night, it got stuck in my junk while she was blowing me last night.
Guy 2: Get out of my house.