Place-names of New Cumnock
Nith: Welsh newydd ‘new, freshness’
The River Nith holds the unique distinction in Ayrshire of being the only major river that does not flow into the Firth of Clyde. Instead after flowing for twelve or so miles from its source in the hills of New Cumnock, all the while feeding itself of the rivers and burns of the parish, it departs Ayrshire at Corsencon hill and enters Dumfriesshire and 40 or so miles downstream empties into the Solway Firth.
In Ptolemy’s map of Roman Britain (c. 140 AD) it appears as Novius fluvius which is said to form the eastern boundary of the Novantae tribe of south-west Scotland.
Early Forms of the Name
Ystrad Nidd?, Stranid (1128), Stranith (1178), Nyddisdaill (c 1400), Nithisdale (1408), Nethis- (1440), Nid(d)is- (1544), Nithsdale.
W.J. Watson  in his assessment of the River Nith supports the general view that ‘Novious is agreed to be, by position, the Nith’,but challenges (intially at least) ‘Whether the name Novious is
represented by modern Nith’. If it is, then the name has its origins in Welsh newydd ‘new, fresh’, which is cognate with the Latin novus ‘new’.
‘Here the term new is assumed to be some reference to the freshness of the river-side which still to this day has some significance. The transformation from newydd to nith no doubts spans British, Welsh, Gaelic and English speaking influence.’
A great place to view the Nith and its fresh green banks is from the March Bridge at the march or boundary between Ayrshire and Dumfriesshire below the slopes of Corsencon hill. Personally , I like the idea that the source of the name Nith is ‘new’, quite fitting for a river that has its source in New Cumnock. A river that also gives rise to the name Cumnock (Gaelic commun achadh ) derived from the meeting place of the River Nith and Afton Water.
 W.J. Watson ‘The Celtic place-names of Scotland’