Welsh people are often referred to as Taff, but could somebody please tell me why?

Most dictionaries say that Taff ot Taffy is a corruption of the common Welsh name Dafydd (David in English).

Taff is an abbreviation of Taffy.

"Taffy comes from the word David, a very common Welsh name. David, familiarly Davy, becomes in Welsh Taffid, Taffy."
http://www.bartleby.com/81/16194.html

"It may be derived from the Welsh forename Dafydd, meaning "David", and is now considered offensive or contemptuous by many people."

https://en.wikipedia.org/wiki/Taffy

Hi, im Lynz from Merthyr Tydfil, in South Wales, which is inbeween the Brecon Beacons and Cardiff. We live in the actual "valleys" that are depicted on t.v, (normaly when taking the mick out of us.)I have always belived that the English call us "TAFFY" because of the river Taff that starts as a mountain stream in Brecon, travels 20miles south through my village,(by now a fast flowing river),and then travels the remaining 25miles down to Cardiff docks and joins the sea. I have never heard that it originates from David, or ever heard any Davids I know(and there's a few around Wales!), whose name is shortened to Taffy,the Welsh dont call Each other Taffy, It'd sound stupid! Any way I'v never been offend by that name,anyway as im looking at the very fast, extremly powerful but agile River Taff right now and I think its the most perfect descrption of the Welsh I'v every heard!!!

For every Saesneg that had heard of the Taff, there were five or more that would think Dafydd was Daffy or Taffy. Just as Shakespeare spelled Llewelyn as Fluellen. Go for the simple explanation. Why would a river in South Wales be used for any Welshman? Dewi Sant (Saint David) is a much more likely source in that he led to the popularity of the name. https://en.wikipedia.org/wiki/Saint_David

It comes from the river taff, as it acted as a natural border alongside the defences made by the Saxons along offas Dyke, dros clawdd offa was often a term used by the Welsh for going to England. It's an English term of derision just like the saxon word welch, meaning foreigner. The Saxons called the native celts this after they were invited over as mercenaries then they like it so here much they pushed the celts back to the celt fringes we see today.