The human hand has evolved a thumb that can not be compared with the ape hand. The human thumb is far more flexible and powerful than that of the ape, which enables human beings to grasp accurately and finally develop the ability to use tools. A thumbs up is a sign of appreciation for a person.
Now more and more people have tattoos, and they will tattoo in many places, not only on the arms and hands that we often see. So here, it is to tattoo their thumbs, many people will feel incredible, but it is true that the thumb tattoo is welcomed by a large number of people.