Yes, nub is a Scrabble word!
...and is worth 5 points in Scrabble, and 8 points in Words with Friends
nub
Noun
1. A small lump or protuberance
2. A small piece
3. The choicest or most essential or most vital part of some idea or experience
What Scrabble words can I make with the letters in nub?
3 Letter Words
2 Letter Words