Placename evidence can often be a revealing source of information, not only about the linguistic and cultural history of the landscape, but also its history, legends and mythology. Lothian, of course, was not always known as Lothian, and the shifting patterns of its names can yield important clues.
To the early Romans in the first century AD, the Lothian region was the Northern frontier of the independent confederation of Brythonic Celtic kingdoms known as Brigantia, which probably had its "federal" capital as far South as West Yorkshire in England, under the high queen Cartimandua. However, while no specific regional placename is indicated in the Latin sources, Lothian itself was inhabited by a Brythonic tribe known in Latin as the Votadini.
For a regional name as such, we have to wait until the post-Roman seventh century and the first surviving native literature from the region, specifically, Aneirin's heroic poem Y Gododdin (c.600). At this point, the tribe known to the Romans as the "Votadini" reveal their actual name as the Gododdin, and this ethnonym is also transferred to the territories which they hold. Hence, the first regional name which we have records of for Lothian is Gododdin. Yet little mythology as such surrounds this earliest of placenames for Lothian. For that, we have to wait until the second millennium, and the roots of the modern placename.
Some early Latin spellings of Lothian and Latinate Norman French are Lodoneo (1098), de Lodoneio (1117), Loeneis (C12th), and Leudonia (1158), Loenes (1249), Leudonia (1164), Laodonia (C12th), etc. [1]. However, a C12th Welsh spelling is Lleudinyawn (modern Lleuddiniawn), in the text of a poem by Hywel ap Owain:
Neu dreitsy tra lliw Lleudinyawn dreuyt
It seems likely that the Welsh form derives from a previous, yet unrecorded Brythonic form along the same lines, which would represent the earliest form, and mean the "land of Lleu's fortress", comprised of the elements lleu (personal name), din (fortress), and -iawn (land of).
These early Latin and Welsh/Brythonic spellings were later turned into the Anglian form Lošene, and Scots forms along the lines of Loyan, Lowthyan, Lothiane, and so forth, finally becoming the modern Lothian.
According to John Garth Wilkinson, Lleudinyawn could derive from an Old Celtic Lugudunion [2]. Other placenames in Northern Britain which seem to follow this root can be conjectured, not least Loudoun (Ayrshire), Lugton (Renfrewshire), and Luguvalium, the Romano-Celtic name for Carlisle in Cumbria (a central city in later Arthurian mythology). However, similar placenames throughout ancient and modern Europe are also conjectured to derive from the likes of Lugudunion and Lugudunum, such the Gaulish placenames Lyon, Loudun and Leiden, and even the name of London is sometimes thought to have this root.
Luggate and the Luggate Burn in East Lothian, nearby the Loth Stane and Traprain Law, are also worthy of mention, not least given Traprain's mythological and legendary association with the figure of Loth in his various guises. Ludgate in London is also often mentioned in connection with the pagan deity sometimes associated with the mythological Loth.
One way or another, it seems likely that the first meaning of Lothian derives from an old p-Celtic root meaning the "land of Lleu's fortress". The question then arises, of course, as to who this Lleu actually was, and how he becomes transformed into the modern Loth. One theory links him in with a Brythonic pagan sun-god, while others suggest an Arthurian knight, a historical king, or a legendary giant.
However, on a purely linguistic level, it seems likely that the transformation of the Celtic personal name Lleu into the modern Scots figure of Loth derives from an incorrect segmentation of the original Brythonic placename Lleuddiniawn, with the initial soft consonant of ddin (a voiced "th" sound) being attached to the end of the lleu element, yielding a spurious lleudd, eventually becoming pronounced in the mouths of Scots speakers as loth.
footnotes
[1] W.J. Watson, The History of the Celtic Placenames of Scotland (Edinburgh: 1926), p.101.
[2] John Garth Wilkinson, West Lothian Place Names (Harburn: 1992), p.16.