Club thumbs is a sign of royal lineage.
A clubbed thumb is a term used to describe the genetic clubbing of one or both thumbs. It is characterized by a particularly short thumb that is round in section and bulbous at the end. The thumb nail is very wide and short.
Clubbed thumbs are historically thought of as a sign of royalty. This rare condition was much more common in royal European blood-lines than in the general population and was used as a factor in determining the pureness of the blood.
https://www.wikidoc.org/index.php/Clubbed_thumb