When something is underwhelming or disappointing. Most often used in a context that involves a person describing the poor state of something, such as a meal or TV show.
"Dude, this caffood is straight buttslap." "I'm bummed bro, that movie was buttslap."
An interjection used to cut someone off mid-sentence and hopefully cause them to lose their train of thought. Should be said in a loud raspy voice at any inappropriate time.
Bob: Hey so this one time i was...
Fred: BUTT SALAD!!!!
Bob: What the?! ...wait what was i saying?
Fred: ...butt salad?