Your wife is entitled to maintenance (nafaqah). It is your obligation to provide for her maintenance. You have to give her her due nafaqah.
If, however, she refuses to give you sex, you are not obliged to give her the nafaqah. The nafaqah you give her is in return for the sex she allows you to have with her.
When she refuses to give you sex she comes under being naashiz. Nushuuz cancels the obligation of giving her her nafaqah.
Nushuuz is the verbal noun of the verb nashaza. Nashaza lexically means to be disobedient. nushuuz, thus, means disobedience or more specially the failure to fulfill the duties laid upon you toward your husband.
Naashiz is the active participle of the verb nashaza. It means the one who shows nushuuz (disobedience).
al-mughni li Ibn Quduumah