Sorry to confuse the thread by answering a February question, but I was brought up in Barogoed (well, Gilfach, actually), and know the answer to this one... It's not what you think...
Bargoed was the name of the railway station on the Rhymney Railway, which was built just above Pont Aberbargod (You can still see it just next to the "new" road bridge at the bottom of Aberbargoed Hill). The new town that got the railways station was called Robertstown at the time, if i remember rightly.
Pont Aberbargod is named for the place where the Bargod Rhymni runs into the Rhymni river.
The Bargod Rhymni stream (which comes from Fochriw, via Deri) contains the word Bargod in the meaning of a boundary, and corresponds to the Bargod Taf which runs down "Cwm Bargod", the next valley over again, but both streams start close together in between Rhymney and Merthyr, and were presumably at one stage considered the boundaries of the Taf and Rhymney areas - possibly in terms of the Medieval cwmwds or similar.
There was also the additional confusion of the south end of the enw town being built on the land of the Gilfach Fargoed mansion, so the railways decided to take a halfway hose between Aberbargod and Golfach Fargod, and call it Bargoed, with the added change of spelling to suit other local hotspots such as Hengoed (pronounced locally as hengod, but actually referring to an old wood).