An Etymological Dictionary of the German Language/neun
neun, num., ‘nine,’ from the equiv. MidHG. and OHG. niun; corresponding to Goth. niun, ‘nine,’ OSax. nigun, Du. negen, AS. nigun, E. nine, OIc. níu (all these represent *nī̆jun?); a common Aryan numeral, like all the units. Comp. Sans. návan, Lat. novem, Gr. ἐννέα, OIr. nói. It has been supposed that the common Aryan word for neun (newn) is derived from neu (néwos), ‘nine’ being regarded as the ‘new number’ of the third tetrad; the system of reckoning by fours must be assumed as the oldest in the Aryan languages, since the numeral acht, ‘eight,’ is a dual form.