Green’s Dictionary of Slang

(Irish) work undertaken in one’s free time, as part of the ‘black economy’; thus nixered adj.

1942		S. O’Faolain Bell V 234: . It suddenly occurs to him, as the hooter goes at one-forty- five, that he has a ‘nixer’ to do for his next-door neighbour.
1950		S. Murphy Stone Mad (1966) 174: Catch us to be without a foxer on a Bank Holiday.
1970		C. Brown Down All the Days 183: Don’t be worrying, Ma. Joe’s in the way of doing a few nixers. He’s doing an extension for Mattie Madigan.
1987		V. Caprani Rowdy Rhymes (2011) [ebook] By payday of that week there were a couple of hundred ‘nixered’ broadsheets of the ballad being passed from hand to hand .
1993		M. Verdon Shawlies, Echo Boys, the Marsh and the Lanes 35: Not being a sawney, he spotted a way for doing foxers and soon he had loads of lops.
2001		G. Coughlan Everyday Eng. and Sl. 🌐 Nickser, Nixer (n): a job done on the quiet so that no tax has to be paid on the wages.
2004		P. McNiff Stories from a Small Town 134: I used to do an odd nixer during the weekend down in the forge. [...] There were a few handy pounds in it and it was easy money.