 Perhaps the reference is to the darkness of the river waters that flow near the town; if so, then the name can be compared to the English Tubney, Tubbanford, Tub Mead and Tub Hole in England.Compare the late Vulgar Latin "tubeta" 'morass', from Gaulish.The root is found in Old Irish "dub" > Irish "dubh", Old Welsh "dub" > Welsh "du", Old Cornish "duw" > Middle Cornish "du", Breton "du" Gaulish "dubo-", "dubis", all meaning 'black; dark'

Second element from Celtic *"magos", 'plain, field', first perhaps related to Old Irish borb 'fierce, violent, rough, arrogant; foolish'

From Celtic *"(φ)erkunos" 'oak' or divine name Perkwunos + Latin "jugum" 'summit'

From Celtic *"brigant-" 'high, lofty, elevated' (or divine name, Brigantia)
Perhaps from Celtic *"genu-" 'mouth [of a river]'.