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.