Yes, sign is a Scrabble word!

Verb

1. Mark with one's signature; write one's name (on)

2. Approve and express assent, responsibility, or obligation

3. Be engaged by a written agreement

4. Engage by written agreement

5. Communicate silently and non-verbally by signals or signs

6. Place signs, as along a road

7. Communicate in sign language

8. Make the sign of the cross over someone in order to call on God for protection; consecrate

Adjective

1. Used of the language of the deaf

Noun

1. A perceptible indication of something not immediately apparent (as a visible clue that something has happened)

2. A public display of a message

3. Any nonverbal action or gesture that encodes a message

4. Structure displaying a board on which advertisements can be posted

5. (astrology) one of 12 equal areas into which the zodiac is divided

6. (medicine) any objective evidence of the presence of a disorder or disease

7. Having an indicated pole (as the distinction between positive and negative electric charges)

8. An event that is experienced as indicating important things to come

9. A gesture that is part of a sign language

10. A fundamental linguistic unit linking a signifier to that which is signified

11. A character indicating a relation between quantities