Start with the surname 'Edgar'. Drop the 'g', and add an equal amount of imagination. There you have it .... Adair. Dropping the 'z' from 'Edzear' requires somewhat less imagination to achieve the same result. This theory states that Adairs were originally Edgars. Perhaps this arose from translation and pronunciation between legal Latin, Gaelic and English.
The first Edgar is thought to be Edgar who held extensive lands in Nithsdale during the reign of William the Lion of Scotland. The Edgars were Gaelic in origin and their name means 'happy spear'. About the year 1200, Edgar granted the churches of Kyllosber and Mortun in Struhtun to the Abbey of Kelso. Edgar's father, Duvenald, was a leader of the Galloway men in the battle of the Standard between King David I of Scotland, and Prince Stephen of Northumbria (Northern England) August 22, 1138. The battle earned its name from the English who had the banners of Thurstan's four Yorkshire saints as inspiration and rallying point for the English troops. Prior to commencement of hostilities, King David planned to put his best equipped troops in the front rank. This offended the men from Galloway of whom the Edgars were part. The Galloway men claimed they had, "iron sides, a breast of bronze and a mind void of fear". This they said made up for the lack of armour. Therefore, they demanded the honour of being first in battle. When Scottish trumpets signaled general advance on the English enemy, the Galloway men "gave vent thrice to yell of horrible sound". As the Galloway men ran across the field toward the English, they banged their spears against their shields to 'psyc out' the foe. English archers, sheltered behind the English front rank of armoured knights, played their own version of psychology by waiting till the Galwegians were close so as to maximize the shock of their first volley of arrows. Although the men of Galloway bravely kept coming, they were cut down, many pierced with several arrows each. They did not come into range for their swords to be of use.
Edgar's grandfather was Donegal or Dunegal of Stranid and Morton Castle. Land on which stands Morton castle, was owned by the Adairs until early in the 14th century. Richard Edgar of Wedderbie, a witness at the second marriage of King Robert Bruce, possessed the castle and half the barony of Sanchar or Seneschar in Upper Nithsdale during the Bruce's reign. This Richard is probably the same Richard Edgar who was sheriff of Dumfries in 1329 and the Ricardus Edger who witnessed a royal charter of the lands of Dalmakeran sometime between 1316-1318.
A descendant of Duvenald, Robert or Thomas Edzear, had a charter from King Robert Bruce of the lands of Kildonan in the Rynes. Kildonan is near Kinhilt, where the Adairs had a stronghold. Thomas Edzear also went by the name Odeir. Several deeds prove the name of Edzear and Adair to be interchangeable with the Adairs of Galloway. In a charter dated 1625 the name is spelled both ways on the same page. Moreover, both Edgar and Adair are septs of the clan Maxwell