"Glendinning" Scots: habitation name from a place in the parish of Westerkirk, Dumfries, Scotland recorded in 1384 as Glendonwyne. It is apparently so called from British ancestors of the words glyn valley, din fort, and gwyn fair, white. Variants: Glendenning, Glendennon, Glendoning, Glendining, Clendenning, Clendenen, Clendennen, Clindening.
The Glendennings are a sept of the Douglas Clan and the history of the name goes back to Adam de Glendonwyn who was alive during the reign of Alexander III of Scotland, circa 1286. Adam is supposed to be a 15th generation descendent of Nomenoe, King of Britanny (841 ad). Glendennings also claimed direct descent. Adam was the youngest of four sons of Hugh FitzHenry Knight of Ravensworth Yorkshire and Aubrey, daughter of Sir William de Steyngrave. He was known as FitzHugh before the Glendonwyn lands were granted to him. His brother, Henry, fought at Bannockburn and was Constable of Barnard's Castle in 1315 and another brother, John of Catton, married Isabel de Rihil, the heiress to the manor of Whittingham in Northumberland.
Educated men in Scotland spoke the language of their allies, the French, and many surnames developed based on place names - de Glendonwyn meaning of (or from) Glendonwyn.
Adam de Glendonwyn's descendents became knights and substantial landholders, fighting alongside the Douglas clan leaders in their battles with the English and were often to be found offering themselves to English Kings as hostage for their countrymen's good behaviour.
The clan grew and ultimately began to spread - across the border to England, over to Ireland during the Plantation of Ulster in 1610 and on to the New Worlds, Scots being leaders in emigration. The most common reasons for the earliest border crossings were raids to steal English sheep, cattle, and horses. Some of these men, known as reivers, just never went home and were eventually accepted by the community they chose to settle in.