The idiom 'I am all thumbs' used to mean that a person is clumsy. Clumsy means that a person is awkward and uncoordinated. If you told your child, "You are all thumbs", this would be considered negative statement, not a compliment.🤕 It would mean they may drop their books frequently, or could not catch a ball.
Teenagers today are changing the meaning of the idiom. Have you watched a young person texting on their smartphone at lightening speed with just their thumbs? The meaning of idiom within the sentence: You are all thumbs could soon become a compliment!
The New York Times article discusses this idiom.
Here are other idioms extracted from the New York Times.