Nikāchit karma is karma that has become firmly bound and fixed, so it cannot be reduced, delayed, or softened by later repentance, austerity, or other efforts.

Simple meaning

• Nikāchit means fixed, settled, or irreversible in result.
• In Jain karma theory, some karma becomes so mature and bound that its fruit must be experienced exactly as bound.
• Such karma does not get modified by ordinary means once it has fully solidified.

Jain understanding

• Karma is bound by thoughts, speech, and actions.
• Normally, through pratikraman, penance, repentance, and restraint, a soul can reduce the bondage of new karma.
• But when karma becomes nikāchit, its fruition is निश्चित — it must give its result.

Example

If a person has bound strong karma for:

• pain,
• suffering,
• lifespan,
• or certain life conditions,

then when that karma becomes nikāchit, its effect will definitely arise at the proper time.

Spiritual lesson

Jainism teaches that we should be very careful before karma becomes firm, because:

• once bound strongly, it becomes difficult to change,
• but by right conduct, vigilance, and non-violence, we can prevent new bondage.

In brief

Nikāchit karma = fully fixed karma whose result is unavoidable.

If you want, I can also explain it in Gujarati/Hindi or in relation to bandh, udaya, and nikāchit bandh.