There is virtually no direct attestation of Pictish, short of a limited number of place names and names of people found on monuments and the contemporary records in the area controlled by the Kingdom of the Picts. However, evidence from place names and personal names points to the language being closely related to the Brythonic languages spoken prior to Anglo-Saxon settlement in what is now southern Scotland, England and Wales.
The existence of a distinct Pictish language during the Early Middle Ages is attested clearly in Bede's early 8th century Historia ecclesiastica gentis Anglorum, which names Pictish as a language distinct from that spoken by the Britons, the Irish, and the English. Bede states that Columba, a Gael, used an interpreter during his mission to the Picts. A number of competing theories have been advanced regarding the nature of the Pictish language:
Most scholars agree that Pictish was a branch of the Brythonic languages. Pictish came under increasing pressure and influence from Old Irish spoken in Dál Riata from the 5th century until its eventual replacement.
Pictish is thought to have influenced the development of modern Scottish Gaelic. This is perhaps most obvious in the contribution of loan words, but more importantly it is thought that Pictish influenced the syntax of Scottish Gaelic, which bears greater similarity to Brythonic languages than does Irish.
Personal names of Roman-era chieftains from the Pictish area, including Calgacus (above) have a Celtic origin.
Celtic scholar Whitley Stokes, in a philological study of the Irish annals, concluded that Pictish was closely related to Welsh. This conclusion was supported by philologist Alexander MacBain's analysis of the place and tribe names in Ptolemy's 2nd century Geographia. Toponymist William Watson's exhaustive review of Scottish place names demonstrated convincingly the existence of a dominant P-Celtic language in historically Pictish areas, concluding that the Pictish language was a Northern extension of British and that Gaelic was a later introduction from Ireland.
William Forbes Skene argued in 1837 that Pictish was a Goidelic language, the ancestor of modern Gaelic. He suggested that Columba's use of an interpreter reflected his preaching to the Picts in Latin, rather than any difference between the Irish and Pictish languages. This view, involving independent settlement of Ireland and Scotland by Goedelic people, obviated an Irish influence in the development of Gaelic Scotland and enjoyed wide popular acceptance in 19th century Scotland, but is no longer given credence.
While Skene's notion of an exclusively Q-Celtic Pictish language has long been rejected, the Picts were under increasing political, social and linguistic pressure from Dál Riata from around the 5th century. The Picts were steadily Gaelicised through the latter centuries of the Pictish Kingdom, and by the time of the merging of the Pictish and Dál Riatan kingdoms, the Picts were essentially a Gaelic-speaking people.Forsyth (1995a) speculates that a period of bilingualism may have outlasted the Pictish kingdom in peripheral areas by several generations.Scottish Gaelic, unlike Irish (and, for that matter, Old Irish) maintains a substantial corpus of Brythonic loan-words and, moreover, uses a verbal system modeled on the same pattern as Welsh.
Traditional accounts (now rejected) claimed that the Picts had migrated to Scotland from Scythia, a region that encompassed Eastern Europe and Central Asia. Buchanan, looking for a Scythian P-Celtic candidate for the ancestral Pict, settled on the Gaulish-speaking Cotini (which he rendered as Gothuni), a tribe from the region that is now modern-day Slovakia. This was later misunderstood by Robert Sibbald in 1710, who equated Gothuni with the Germanic-speaking Goths.John Pinkerton expanded on this in 1789, claiming that Pictish was the predecessor to Modern Scots. Pinkerton's arguments were often rambling, bizarre and clearly motivated by his belief that Celts were an inferior people. The theory of a Germanic Pictish language is no longer considered credible.
Difficulties in translation of Ogham inscriptions, like those found on the Brandsbutt Stone, led to a widely held belief that Pictish was a non-Indo-European language
John Rhys, in 1892, proposed that Pictish was a non-Indo-European language. This opinion was based on the apparently unintelligible ogham inscriptions found in historically Pictish areas. A similar position was taken by Heinrich Zimmer, who argued that the Picts' supposedly exotic cultural practices (tattooing and matriliny) were equally non-Indo-European, and a Pre-Indo-European model was maintained by some well into the 20th century.
A modified version of this theory was advanced in an influential 1955 review of Pictish by Kenneth Jackson. Jackson proposed a two-language model: while Pictish was undoubtedly P-Celtic, it may have had a non-Celtic substratum and a second language may have been used for inscriptions. Jackson's hypothesis was framed in the then-current model that a Brythonic elite, identified as the Broch-builders, had migrated from the south of Britain into Pictish territory, dominating a pre-Celtic majority. He used this to reconcile the perceived translational difficulties of Ogham with the overwhelming evidence for a P-Celtic Pictish language. Jackson was content to write off Ogham inscriptions as inherently unintelligible.
Jackson's model became the orthodox position for the latter half of the 20th century. However, it has become progressively undermined by advances in understanding of late Iron Age archaeology, as well as by improved understanding of the enigmatic Ogham inscriptions, a number of which have since been interpreted as Celtic.
Despite this, Eric P. Hamp in his 2012 Indo-European family tree, classified Pictish as a non-Indo-European language.
^Nordhoff, Sebastian; Hammarström, Harald; Forkel, Robert; Haspelmath, Martin, eds. (2013). "Pictish". Glottolog 2.2. Leipzig: Max Planck Institute for Evolutionary Anthropology.
^Bede HE I.1; references to Pictish also at several other points in that text.
^All other research into Pictish has been described as a postscript to Buchanan's work. This view may be something of an oversimplification: Forsyth (1997) offers a short account of the debate; Cowan (2000) may be helpful for a broader view.
^Proceedings of the Second International Congress of Celtic Studies 1963, p135
^See for example Bede HE I:1; Forsyth (2006) suggests this tradition originated from a misreading of Servius' fifth century AD commentary on Virgil's Aeneid. Aeneid 4:146 reads: Cretesque Dryopesque fremunt pictique Agathyrsi. Servius' commentary states: Pictique Agathyrsi populi sunt Scythiae, colentes Apollinem hyperboreum, cuius logia, id est responsa, feruntur. 'Picti' autem, non stigmata habentes, sicut gens in Britannia, sed pulchri, hoc est cyanea coma placentes. Which actually states that the Scythian Agathyrsi did not "bear marks" like the British, but had blue hair.
^See Armit (1990) for an up-to-date view of the development of proto-Pictish culture and Brochs as an indigenous development; Forsyth (1998) gives a general review of the advances in understanding of Ogham.