Straight thumb – when your thumb is firm

The person is able to undertake tasks with a sense of purpose and responsibility. He or she is very reliable, serious and careful. If the thumb is extremely stiff, the person can be rigid and may be hindered by too many rules.

Crooked thumb – when your thumb is flexible

People with a flexible thumb are generous and spontaneous. They are willing to change their schedule in order to help others. Because rules are not important to them, they may lack discipline and persistence. They can be less reliable or not reliable at all!

Source: https://glowpink.com/blog/there-are-2-types-of-thumbs-choose-yours/