Masturbating just before sleeping. Similar to the way a nightcap can help you relax and unwind at the end of the day before sleeping. Common in young males that have high testosterone levels and need a way to shoot off some steam. Can also be useful in situations of extreme horniness making you unable to sleep.
"I was way too stressed to sleep, so I had a nightfap and slept like a baby."
"Man I was hanging around Julia all day, so I was way too horny to sleep. Nightfap solved it."