Nusyuz in language is non-compliance, taken from an-nasyz which means high ground, non-compliance is called nusyuz because the perpetrators feel higher so that he does not feel the need to obey.
Nusyuz in household terms is the dislike of husband and wife to his spouse. The woman is nusyuz to her husband if she does not obey her, husband nusyuz to wife if she treats her badly and turns away from her.