Yes, mum is a Scrabble word!
...and is worth 7 points in Scrabble, and 10 points in Words with Friends
mum
Adjective
1. Failing to speak or communicate etc when expected to
Noun
1. Of China
2. Informal terms for a mother
3. Secrecy
What Scrabble words can I make with the letters in mum?
3 Letter Words
2 Letter Words