That's considered underweight.
BMI:
Below 18.5 | Underweight |
18.5 - 24.9 | Normal |
25.0 - 29.9 | Overweight |
30.0 & Above | Obese |
I can't imagine how flabby your arms could be. But if you do want to tone them, I recommend:
http://www.fitnessmagazine.com/workout/arms/exercises/tone-your-arms-in-3-moves/