Yes, weak is a Scrabble word!

Adjective

1. Wanting in physical strength

2. Overly diluted; thin and insipid

3. (used of vowels or syllables) pronounced with little or no stress

4. Wanting in moral strength, courage, or will; having the attributes of man as opposed to e.g. divine beings

5. Tending downward in price

6. Deficient or lacking in some skill

7. Lacking bodily or muscular strength or vitality

8. (used of verbs) having standard (or regular) inflection

9. Not having authority, political strength, or governing power

10. Deficient in magnitude; barely perceptible; lacking clarity or brightness or loudness etc

11. Likely to fail under stress or pressure

12. Deficient in intelligence or mental power