An examination of the public record shows that the name MacKay was spelt in a great variety of ways. The Strathnaver or northern MacKays appear as Makky, Macky, Maky, Mckye, Mckeye, Mackie, Mckie, Mackey, but the commonest form was McKy. The Islay MacKays, whose Charter in Gaelic, of eleven and a half merk lands from Macdonald of the Isles in 1408, is well known, appear as McCei, McAy, etc. The MacKays of Garachty in Bute, one of whom, John McGe, witnessed a document [Reg.Mag.Sig.], 10th Mar 1540, as Sheriff of Bute, appear as Makkay, Makkee, and even Makcawe, but for oftenest they are represented, from 1515 downwards, as Mackaw. The MacKays of Ugadale, who were crowners of north Kintyre from time immemorial, held of the Lords of the Isles as is stated in a charter of confirmation by the King, [Reg.Mag.Sig.] given 11th Aug 1542, and possessed of the four merk lands of Ugadale and Arnigill in virtue of their office, appear as McKey, MaKKay, MaKKaye, but most commonly as McCay. The Galloway MacKays, of whom there were various families holding a considerable amount of land in Wigton, Kirkcudbrightshire, etc., such as Camlodane, Balgarne, Craichlo, Mertoun, Balmagee, etc., appear as Makke, Makee, Makge, Makgee, Makgie, McGie, McGhie, and Mackghie, but towards the close of the 16th century they appear generally as McKie and McGhie.
MacKay represents in English the Gaelic name MacAoidh, a compound of mac (son), and Aoidh the genitive of the proper name Aodh. Aodh frequently appears in the literature of the Gael as the name of Picts, Scots, and Irish; but its present aspirated form indicates a harder formation, AED, which indeed is found in earlier Irish writings, and is supposed to mean "the fiery or impetuous one". [Gaelic Ety Dictionary] Some authorities have equated Aodh with Hugh, but we do not accept that view, as Hugh, which stands for the Gaelic Huistean, is generally represented in Latin documents by Hugo, while Aodh is transformed into Odo or Odoneus. Nay more, there are various instances in which two brothers may be found, the one called Aodh and the other Hugh, as, for example, the family of Donald 1st Lord Reay, whose first and third sons were so named respectively. This shows that they were considered two different names then, just as is the case to-day among Strathnaver people. Probably the best English equivalent of Aodh is Iye, if it can be called an equivalent; and it is a pity that this name, as characteristic of the MacKays once as Rorie is of the Macleods or Ranald of the Macdonalds, is not more commonly used nowadays.
[Footnote: Among Strathnaver people at the present day a person addressed as Iye, in Gaelic, signs his name and is addressed, in English, as Isaac. The surname MacIsaac, to be found in the West Highlands, may be a corrupt form of Mackay.]
Like the name MacKay. Aodh also has been twisted into a great variety of forms to suit the fancy of different writers. In the Earldom of Sutherland, written about 1630 by Sir Robert Gordon, the name is spelt Iye, and in some cases Y simply. In the 1415 charter by the Lord of the Isles to Angus Du of Strathnaver, it is spelt Eyg, and in the will of Sutherland of Dunbeath, dated 1456, it is spelt Aytho.
[Footnote: "Item.- I geve and assigns to my douchtir Marion al the lave of my landis that I have undisponyt upone, and sa mony ky ald and zong as I have wyth Aytho Faurcharsone, or wyth MacKay Benauch [Bonar] and sa mony ky as scho aucht to have of William Polsony's ky." - Miscellany of the Bannatyne Club. Vol. III.]
"Mariota filia Athyn", the first wife of the "Wolf of Badenoch" and the mother of his children, was a daughter of Athyn, another form of the name Aodh. In some of the Latin documents included in the Chronicles of the Picts and Scots, in Fordun's Annalia, etc., Malcolm MacEth or MacKay, who claimed the earldom of Moray, and became first Earl of Ross about 1157, is variously denominated MacEth, MacHeth, MaeEd, MacHead, etc. Dr. Macbain, who edits the second edition of Skene's Highlanders of Scotland, writes at page 414 - "The name Heth is the most ill-used syllable I know of. It appears as Head, Ed, Eth; the Gaelic form of all these monstrosities can easily be identified. It is the very favourite name of Aed or Aodh, later translated as Hugh. Macheth is an old form of MacKay."