Yes, cos is a Scrabble word!
...and is worth 5 points in Scrabble, and 6 points in Words with Friends
cos
Noun
1. Ratio of the adjacent side to the hypotenuse of a right-angled triangle
2. Lettuce with long dark-green leaves in a loosely packed elongated head
3. An odorless very poisonous gas that is a product of incomplete combustion of carbon
4. A hard ferromagnetic silver-white bivalent or trivalent metallic element; a trace element in plant and animal nutrition
5. One who refuses to serve in the armed forces on grounds of conscience
6. A state in west central United States in the Rocky Mountains
What Scrabble words can I make with the letters in cos?
2 Letter Words