Yes, yak is a Scrabble word!
...and is worth 10 points in Scrabble, and 9 points in Words with Friends
yak
Verb
1. Talk profusely
Noun
1. Noisy talk
2. Large long-haired wild ox of Tibet often domesticated
What Scrabble words can I make with the letters in yak?
3 Letter Words
2 Letter Words