the finsterwallies are Violent trembling of the body, especially in the extremities (arms and legs)
The exact definition from the book is: finsterwallies - violent trembling of the body in the arms and legs.
The Finsterwallies are a rough and tough group of boys who live in the East End of Two Mills in the book "Maniac Magee." They are known for causing trouble and intimidating others in the neighborhood, including the protagonist, Jeffrey Lionel Magee.
Violent trembling of the body, especially in the extremities(arms and legs)