Here is Rule #11 from George Washington's Rules for Decency and Civil Behavior, and its translation:
|11th||Shift not yourself in the Sight of others nor Gnaw your nails.|
In other words, when you are where someone can see you, sit still (do not squirm or shift), and do not chew on your nails.
I shift when I need to in someone else's presence; I need to when my back hurts. I guess one might infer, by part 2, that George thought it was okay to "attend" to your nails by chewing, just not when someone was watching...?