Anglo-Saxon Chron. (Tiber. B.i) anno 1049, se cing þa ⁊ eall here cwædon Swegen for niðing. (Then the king and all the army proclaimed Sweyne an outlaw.)
1956 R. SUTCLIFF Shield Ring iv. 39 You know how hard it goes with me to play the nything.
1999 Sarasota (Florida) Herald-Tribune (Nexis) 27 May A2 On the perjury and obstruction of justice articles, five and 10 niddering Republican senators, respectively, conspired to obstruct honest judicial closure.
It's borrowed from "early Scandinavian", as in Old Norse níðingr "villain" (OED), from Proto-Germanic *nīþa- "animosity", from Proto-Indo-European *nei- "to be excited, shine" (AHD).
*nei- perhaps became Persian نیل nīl "blue, indigo", altered to līl and līlak "bluish". This was borrowed into Arabic as ليلك līlak, then Spanish as lilac, a shrub with bluish flowers.
It's tempting to connect Persian nīl or the Sanskrit cognate nīla- "dark blue" with Greek Νεῖλος, English Nile, but as far as I can tell no connection has been found.