Yes, boos is a Scrabble word!
...and is worth 6 points in Scrabble, and 7 points in Words with Friends
boos
Verb
1. Show displeasure, as after a performance or speech
Noun
1. A cry or noise made to express displeasure or contempt
What Scrabble words can I make with the letters in boos?
3 Letter Words
2 Letter Words