Soulless hands

Most gripplis in Oceasile believe that their souls reside in their thumbs. Losing one is considered unlucky. Losing both, however, is often considered a tragedy. In some areas, people without both their thumbs are repudiated and not welcomed into their clans. Other clans, use the thumbs' connection to the soul to sentence people, removing or blocking their soul, and condemning them until they right their wrongs.

Losing one's thumb(s)

For gripplis, whether you lose only a thumb, the whole hand, or even the whole arm, it is the thumb's loss that they feel the most.
Glyph inscribers are one of the professions most at risk of losing their thumbs, as a badly engraved glyph can result in a magical explosion. These gripplis often wear rings with protective enchantments on their thumbs to try and lessen the chances of hurting their thumbs badly.
The method in how one loses a thumb, however, is irrelevant. How many thumbs you have lost, is not.
A grippli without both their thumbs will be pitied at best, cast out at worst. Usually, particularly if they didn't only lose their thumbs, some mercy may be found. But if by some designs, a grippli were to lose both thumbs, and the rest of their fingers are still present, they will most likely be cast off. It is seen as a sign from the gods that this person has forfeited their soul.
Similarly, a travelling grippli without thumbs cannot expect any camaderie or hospitality. Whether they lost their thumbs in an honourable way, or they were removed as a judgement, most gripplis will not give them the chance to explain.

Why the thumbs?

Gripplis have five digits in their upper extremities, and four in their lower extremities.
It is possible that the origin of these beliefs lies in the distinction between both their hands and feets.