"material of which macadamized pavement is made," 1826, earlier as an adjective (1824), named for its inventor, Scottish civil engineer John L. McAdam (1756-1836), who developed a method of leveling roads and paving them with gravel and outlined the process in his pamphlet "Remarks on the Present System of Road-Making" (1822). Originally the word meant road material consisting of a solid mass of stones of nearly uniform size laid down in layers (McAdam did not approve of the use of binding materials or rollers). The idea of mixing tar with the gravel began 1880s.
"process of laying roads according to the system of John L. McAdam;" 1824, noun of action from macadamize (see macadam).
Australian evergreen tree, commercially important for its edible nut, 1904, from Modern Latin (1858), named for Scotland-born chemist Dr. John Macadam, secretary of the Victoria Philosophical Institute, Australia, + abstract noun ending -ia.