Basically something that disgusts you, find embarrassing or unappealing (mostly) about the opposite sex.
1. Lisa: "So, my boyfriend and I had sex for the first time last night."
Natalie: "Really!? Oh my god, how was it?"
Lisa: "It was okay... but when he started climaxing, he farted! It smelled so bad!"
Natalie: "Ew!! What a turn off!"
2. "There was this random girl who danced with me at the party last night. She was so hot. But when she grinned at me at me, I noticed she was missing two front teeth. Immediately turned me off, man."
1. To fail at something, to do something wrong/incorrectly (sometimes repeatedly). Someone that screws up a lot is called a "screw-up"
2. To describe hurting someone emotionally or physically. Commonly used when threatening the one you want to hurt.
Person 1: "I stayed up late last night to study but I couldn't concentrate that well."
Person 2: "You're gonna screw up that test."
"This is the 5th time in a row I'm getting a bad grade. I'm really screwing up."
"I couldn't stop stuttering when introducing myself to him. I screwed up so badly."
"Oh yeah!? You wanna mess with me, kid!? Ohh, I'm gonna SCREW YOU UP!"
Person 1: "Haha, I totally screwed that kid up, did you see his black eye?"
Person 2: "Yeah, he deserved it!"