When a group of women get together to complain about men, problems with men, or the lack of men.
Usually involves acting in a manner in which any man who sees them would sever all ties with said women, should that man know them.
Things were going great with Mark until Gina and her friends went on a girl's night out.
Mark saw Gina at a club and slit his wrists in the bathroom.
The truly ultimate responce to any kind of stupid or annoying "why" question, or simply to a question you cannot answer.
That answer will totally piss off annoying kids who ask too many questions.
See also: because you're gay.
Kid: Why did the dinosaurs die out?
Tour guide: Because you touch yourself at night.
Son: Why do you drink so much, dad?
Dad: Because you touch yourself at night!
Little brother: Why don't you ever let me into your room?
Big brother: Because you touch yourself at night, you little brat!
Much like your mom and that's what she said, this phrase has a high potential to add innuendo to a previously-uttered statement. Can also be used to augment a comeback.
Person 1: Dude, you suck.
Person 2: No, YOU suck. At night.
Girl 1: So, how are things between you and your boyfriend?
Girl 2: Great! I've never met anyone so awesome!
Asshole 1: At night, that is.
Person 1: I like chillin' with my homeboys. At night.