Yes, foul is a Scrabble word!

Verb

1. Hit a foul ball

2. Make impure

3. Become or cause to become obstructed

4. Commit a foul; break the rules

5. Spot, stain, or pollute

6. Make unclean

7. Become soiled and dirty

Adjective

1. Highly offensive; arousing aversion or disgust

2. Offensively malodorous

3. Violating accepted standards or rules

4. (of a baseball) not hit between the foul lines

5. (of a manuscript) defaced with changes

6. Characterized by obscenity

7. Disgustingly dirty; filled or smeared with offensive matter

8. Especially of a ship's lines etc

Noun

1. An act that violates the rules of a sport