Yes, sup is a Scrabble word!
...and is worth 5 points in Scrabble, and 7 points in Words with Friends
sup
Verb
1. Take solid or liquid food into the mouth a little at a time either by drinking or by eating with a spoon
Noun
1. A small amount of liquid food
What Scrabble words can I make with the letters in sup?
3 Letter Words
2 Letter Words