Toponymy
Toponymists have had difficulty explaining the origin of 'Tintagel': the probability is that it is Norman French as the Cornish of the 13th century would have lacked the soft 'g' ('i/j' in the earliest forms: see also Tintagel Castle). If it is Cornish then 'Dun' would mean Fort. Oliver Padel proposes 'Dun' '-tagell' meaning narrow place in his book on place names. There is a possible cognate in the Channel Islands named Tente d'Agel, but that still leaves the question subject to doubt.
The name first occurs in Geoffrey of Monmouth's Historia Regum Britanniae (c. 1136, in Latin) as Tintagol, implying pronunciation with a hard sound as in modern English girl. But in Layamon's Brut (MS Cotton Otho C.xi, f. 482), in early Middle English, the name is rendered as Tintaieol. The letter i in this spelling implies a soft consonant like modern English j; the second part of the name would be pronounced approximately as -ageul would be in modern French.
An oft-quoted Celtic etymology in the Oxford Dictionary of English Place-Names, accepts the view of Padel (1985) that the name is from Cornish *din meaning fort and *tagell meaning neck, throat, constriction, narrow (Celtic *dūn, "fort" = Irish dún, "fort", cf. Welsh dinas, "city"; *tagell = Welsh tagell, "gill, wattle").
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