Hidden beneath the etymology of the placename of Lothian is a shadowy mythological figure called Lleu. The earliest known Brittonic (and thus linguistically indigenous) form of "Lothian" is from a C12th poem by the Welsh bard, Hywel ap Owain:

Here, Lleudinyawn can be understood in the modern form Lleuddiniawn, meaning, literally, the "land of Lleu's fortress", comprised of the elements lleu (personal name), din (fortress), and -iawn (land of). The question then arises that if the meaning of "Lothian" is "land of Lleu's fortress", who is this Lleu?

Scattered throughout the margins of the Celtic traditions from both Continental and Insular Europe are a series of clues about a pagan sun deity whose name generally means "light". In the Brythonic traditions related to the earliest Celtic culture of Lothian, there are traces of a solar deity called Lleu. In the Gaelic traditions, we have a mythological figure called Lúgh, and in Continental Gaul and Romano-Celtic cultures, he appears to be known as Lugus.

"Lugh's Enclosure"
— in Charles Squire, Celtic Myth & Legend: Poetry & Romance (London: Gresham, nd.)

The majority of what is known about Lleu/Lúgh/Lugus as a pagan deity is generally from the Gaelic tradition, rather than the Brythonic, but one medieval Welsh text with strong Northern associations gives us a certain amount of information about him. Y Mabinogi, transcribed by Welsh monks around the C11th (historically later than the Gaelic texts concerning Lúgh), shows a distinct Christianisation of what were, evidently, nonetheless originally pagan myths. In the story of Mâth ap Mâthonwy, we have a basic version of Lleu's origins. One has to bear in mind, however, that despite the fact that many of the elements in the Mabinogi may have an originally Northern provenance, by the time that the stories were committed to writing, Brythonic-speaking territory had been pretty much reduced to Wales, Cornwall, and Cumbria, although Strathclyde was still a Brythonic-speaking territory on the immediate Western borders of Lothian. As a result, the Welsh scribe who worked on this text tended to set the myths in a local Welsh geography, as so often happens with local variants of otherwise international myths. As a result, the possible original Northern provenance is lost, and Lothian is not specifically mentioned. However, given that Lleu's cult was evidently followed throughout the entire continent, as well as Britain and Ireland, it's no doubt that the myth came in as many versions as there were regions in which it was told. Unfortunately, we have precious little left of the version(s) which may have been told in Lothian itself, and so the later version recorded in Y Mabinogi is probably as close as we can come to it.

"Blodeuwedd's Invitation to Gronw Pebyr"
— in Squire, Celtic Myth & Legend

In Y Mabinogi, Lleu has a magical, apparently virgin birth, and his mother is given as Arianrhod, the ancient Brythonic moon-goddess whose name means, literally, "silver circle", a name which still attached itself to the moon in Welsh lore until recently. More specifically, Arianrhod's mother is Dôn, often thought of as the Brythonic Earth-goddess whose Gaelic equivalent is Danu), which makes Lleu the sun-god the son of Arianrhod the moon-goddess, and the grandson of the Earth-goddess: Earth → Moon → Sun. In Mâth ap Mâthonwy, the miraculous birth of Lleu is given as follows:

Of course, the "some small form" which Arianrhod dropped, unnoticed to all but Gwydion (and presumably the shamed Arianrhod herself) was Lleu. Gwydion then makes arrangements for Lleu to be nursed by a foster-mother.

Here, Lleu is represented as a magical child who grows at twice the normal speed, suggesting giant-like qualities, and intriguingly, this links into another minor origin myth about Lothian, that it is named after a giant called Loth. But in Y Mabinogi, eventually Gwydion takes the boy to Caer Arianrhod (Arianrhod's City, sometimes associated in folklore with the Milky Way) to confront her and ask for her recognition of him. But Arianrhod's shame over the birth of the child leads her to place a threefold curse on the boy, that he will never bear a name, weapons, or a wife. Predictably, Gwydion tricks Arianrhod on each account, but the issue of most interest given our current topic is that of the boy's name:

And so Lleu was given a name by his mother despite her attempts to disown and curse him. Yet a scribal error in Y Mabinogi (accidental or deliberate) gives his name as Llew, meaning "lion", as in the passage quoted above. The generally-accepted correct spelling of his name is in fact Lleu, a Brythonic word meaning, as the verb lleu, "lighted", which gives us modern Welsh goleuo ("to light"), and, significantly, lleuad ("moon"). This series of luminary associations suggests a status as a solar deity, and identifiable as the same gods we find in Gaelic paganism as Lúgh and Gaulish as Lugus. Therefore, taking the full name given in the Mabinogi and correcting it, we have Lleu Llaw Gyffes, meaning "Light Deft-Hand", which has its counterpart in Gaelic versions of this myth in the name of Lúgh Lamh-fada, meaning "Light Long-Hand".

"Lugh's Magic Spear"
— in Squire, Celtic Myth & Legend

Today, Lleu's festival is associated in the Gaelic traditions (in his incarnation as Lúgh) with the first of August, celebrated in Gaeilge (Irish Gaelic) as Lúnasa, and in Gŕidhlig (Scottish Gaelic) as Lůnasdal, both from the older Gaelic form, Lúghnasadh. And as is so often the case with pagan festivals, it has also come down to us as a Christian festival in Scots as the Quarter Day of Lammas. However, appealing though it might be to imagine some etymological link between the Gaelic forms of the word and the name of the festival in Scots, Lammas has purely Germanic roots, deriving from the Anglo-Saxon hlafmæsse, meaning "Loaf-Mass", a Christian festival still held annually in some towns, not least in Kilbarchan, Renfrewshire, to the West of Lothian.

But given the date of the pagan form of the festival, Lúgh or Lleu is clearly a god of harvest-time, as well as being a solar deity. Unfortunately, within the Brythonic tradition, there is no direct recorded equivalent of Lůnasdal, and it may be that this was a primarily Gaelic festival. Furthermore, within the Gaulish tradition as recorded in the Coligny Calendar, August seems to equate to seasons called Elembiuos or Edrinios, which tells us nothing about Lleu or Lúgh's association with this time of the year on the Continent. Sigificantly, though, the Gaulish Lugus' festival was recorded as being celebrated at Lyons in Roman times, and several coins minted in that city show Lugus as the city's patron.

The name of Lothian, then, deriving from a Brythonic form such as Lleuddiniawn, means "land of Lleu's fortress". Furthermore, it seems that the Lleu in question was a Brythonic solar and harvest deity within the pantheon of Celtic paganism throughout ancient Europe, both Continental and Insular, with his festival celebrated on August 1st.

That Lothian should trace its roots to a pagan deity is not unusual. Indeed, within origin myths the world over, it is common for nations, tribes, and elites to claim pseudo-historical and genealogical roots amongst the myths of the gods. Gaelic nobles often traced their genealogies back to the mythical warrior Fionn mac Cumhail, while Anglo-Saxon nobles liked to claim the war-god Woden as their ancestor. In numerous cases, this type of origin myth also survives as a form of creation myth, with the deity in question actually forming the land, as well as begetting the first progenitors of its inhabitants.

Unfortunately, all we can now say regarding Lothian is that its etymology suggests an origin myth concerning the Brythonic deity Lleu. What the specifics of that origin myth were, and what tales were told of Lleu's connection with Lothian in centuries past, we cannot at present know.