According to Spencer, the term amixsak refers to any skin covering, such as the covering of an umiak or kayak. A modern Yupik dictionary gives amiq as meaning “skin” and amirkaq as a sealskin ready for use; the latter may be a more correct term.
When hunting a walrus, it is traditional to butcher the carcass on the ice and take as much as possible back home. If any amount of meat and skin has to be abandoned, the carcass must be given fresh water to drink and the skin must be dissected. If the skin is left behind on the ice, it will sink and become an amixsak, a vengeful monster. An amixsak will come up under an umiak, reach its flippers over the gunwales, and pull the boat under.
Removing the skin covering the flippers on a carcass prevents this danger.
References
Jacobson, S. A. ed. (2012) Yup’ik Eskimo Dictionary, v. I. Alaska Native Language Center, University of Alaska, Fairbanks.
Spencer, R. F. (1959) The North Alaskan Eskimo; a study in ecology and society. United States Government Printing Office, Washington.