NaShrinK is a compressed archive utility, and its associated .NSK file format. It was developed by Nashsoft Systems of Bangalore, India. It is DOS software, and is largely menu-driven. It evidently uses PKWARE DCL Implode compression.
NaShrinK seems to have been part of a software package named NashPack, described as "a systems-maintenance and data-recovery program".
Files start with ASCII "