Yes, cos is a Scrabble word!

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