The story
The term derives from the Latinised Celtic term Mons Graupius, 'hill of the tree', used in Tacitus' Life of Agricola as the location of a battle between the Romans and Caledonians in 83/4 AD. Due to a manuscript reading error Graupius was reproduced as Grampius, and the term was applied to the mountains (formerly called 'the Mounth') in eastern Scotland where the battle was at the time thought to have taken place.
Etymology from Wiktionary, CC BY-SA 4.0.