"nixer" means: A job or income which is in addition to one's normal employment, generally done in the evening or on weekends; ''originally, work for paymen.... This is informal slang, common in casual speech, texting and social media, but not appropriate for school work, applications or professional settings. There is no real cause for concern in itself; it is everyday peer vocabulary. If your child uses it, a light comment about audience and register is usually enough — no need to escalate. Context, more than the word, tells you whether to follow up.