Rother | Toponymy | Forum

Rother, East Sussex.
Of the river Rother in East Sussex Wikipedia (19/12/20) says: “It was known as the Limen until the sixteenth century.” This is not true. The Limden is another tributary which joins the Rother at Etchingham. The name change applies only below this confluence. The Rother rises at between Rotherfield and Castle Hill, So Rother can be parsed as Common Brythonic -duβr, meaning 'water' and Pictish/Brythonic “Roth” (as in Rothiemurchus) cognate with Irish Gaelic “rath” and various places in “-worth” (by Anglo-Saxon metathesis).

The castle was probably some sort of hill fort guarding a useful overnighting spot for drovers - it is just under 20 miles or two days’ droving from several places on the south coast and less than 12 miles from Tonbridge which I suppose to be the next stop to the north.

Rother, Yorkshire etc.
Rotherham takes its name from its location where the river Rother flows into the Don.
Wikipedia is correct in regarding the second bit of Rother being “water” - but it says “The first element could be rö-, an intensive Brittonic prefix meaning 'great', or rūδ, 'red, reddish-brown'.”. This too is “roth” as discussed above, probably referring to Chesterfield where it is recognised that the Romans had a fort - but I suppose that this was built over/next to a Brythonic defence work of some sort.

Rother, West Sussex
I find the suggestion that the name of this river takes its name from Rotherbridge Farm (which does not even have a bridge!) risible.
This River Rother runs right beside St Ann's Hill, Midhust. Despite the name St Ann, the church in Midhurst is dedicated to St Mary Magdalene suggesting serious age. Although the (now ruined) castle on St Ann's hill is credited to the de Bohun family I have no doubt that this too was built on an old Brythonic "roth" from which this river - like the other two - takes its name. Like Rotherfield 'Castle' St Ann's Hill is well placed to assist travellers.