To masturbate when a man gets a boner, but he wasn't particularaly looking to get a boner. So, he must masturbate, or fap, in order to avoid the dreaded blue balls.
I was just watching TV, minding my own business, when I stumbled across a movie set on a beach with a gorgeous blonde, 36-24-36, jogging around in a too-small bikini with her nipples poking out, tits shaking like water balloons. Naturally, I popped a boner. But, since I'm single, there was no one home to satisfy me. Angrily, I said to myself, "I guess I'll have to fap it out. I'm really not in the mood for blue balls tonight."
When you see a sexy girl (e.g. at the YouTube) and you are single, so there is no other way than to fap.
Teenager (coming home from school): Hey dad, my classmate Jill was really hot today!
Dad: Aww, you must be horny! Go to your room quickly and fap it out!