1) a late night smoke session, specifically with the intentions of going to bed immediately following said session.
2) smoking before you go to bed, to aid with sleeping. (potheads swear by it)
can be a personal smoke, (to your face) or a social smoke with some friends.
"Yea man, I'm gonna save half this L for a nightcap."
"I'm goin to bed soon, but I got that goooood, you should stop by for a quick nightcap."