nordic-drifter:

A Selkie is a creature from Scandinavian and Scottish folklore that takes the shape of a seal. However, the seal is able to shed its skin and appear human.

If one were to hide the Selkie’s seal skin it would render them unable to transform back. Selkies are only allowed to make contact with a human for a short period of time, after which they may not see the human again for seven years. But this can be avoided by stealing their skin to prevent them from transforming. In many stories this is the case; a man will steal and hide a female Selkie’s seal skin in order to have control over her. If the Selkie later finds her skin she will return to the sea, abandoning her husband and often her children as well.