I gave my sister a compliment and said she's pretty, then while she was saying thanks, I said, "pretty ugly."
Me: You are pretty. Her: Thanks. Me: Pretty ugly.
Alright listen up buttercup. So, basically, this dude thought he was slick by giving his sister a compliment, but then he hit her with the "pretty ugly" combo. It's like saying something nice and then immediately taking it back with an insult. You probably do this a lot and wonder why people hate you.