Yes, ab is a Scrabble word!
...and is worth 4 points in Scrabble, and 5 points in Words with Friends
ab
Noun
1. A bachelor's degree in arts and sciences
2. The eleventh month of the civil year; the fifth month of the ecclesiastical year in the Jewish calendar (in July and August)
3. The muscles of the abdomen
4. The blood group whose red cells carry both the A and B antigens
What Scrabble words can I make with the letters in ab?
2 Letter Words